drivers/net/virtio_net.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.
Use the new TRAILING_OVERLAP() helper to fix the following warning:
drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
This helper creates a union between a flexible-array member (FAM)
and a set of members that would otherwise follow it (in this case
`u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
overlays the trailing members (rss_hash_key_data) onto the FAM
(hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after `struct virtnet_info` (no
blank line in between).
Notice that due to tail padding in flexible `struct
virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
(at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
offset 84 in struct virtnet_info) are misaligned by one byte. See
below:
struct virtio_net_rss_config_trailer {
__le16 max_tx_vq; /* 0 2 */
__u8 hash_key_length; /* 2 1 */
__u8 hash_key_data[]; /* 3 0 */
/* size: 4, cachelines: 1, members: 3 */
/* padding: 1 */
/* last cacheline: 4 bytes */
};
struct virtnet_info {
...
struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
/* XXX last struct has 1 byte of padding */
u8 rss_hash_key_data[40]; /* 84 40 */
...
/* size: 832, cachelines: 13, members: 48 */
/* sum members: 801, holes: 8, sum holes: 31 */
/* paddings: 2, sum paddings: 5 */
};
After changes, those members are correctly aligned at offset 795:
struct virtnet_info {
...
union {
struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
struct {
unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
u8 rss_hash_key_data[40]; /* 795 40 */
}; /* 792 43 */
}; /* 792 44 */
...
/* size: 840, cachelines: 14, members: 47 */
/* sum members: 801, holes: 8, sum holes: 35 */
/* padding: 4 */
/* paddings: 1, sum paddings: 4 */
/* last cacheline: 8 bytes */
};
As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
moved to the end, since it seems those three members should stick
around together. :)
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
This should probably include the following tag:
Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
but I'd like to hear some feedback, first.
Thanks!
drivers/net/virtio_net.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 975bdc5dab84..f4964a18a214 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -425,9 +425,6 @@ struct virtnet_info {
u16 rss_indir_table_size;
u32 rss_hash_types_supported;
u32 rss_hash_types_saved;
- struct virtio_net_rss_config_hdr *rss_hdr;
- struct virtio_net_rss_config_trailer rss_trailer;
- u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
/* Has control virtqueue */
bool has_cvq;
@@ -493,7 +490,16 @@ struct virtnet_info {
struct failover *failover;
u64 device_stats_cap;
+
+ struct virtio_net_rss_config_hdr *rss_hdr;
+
+ /* Must be last --ends in a flexible-array member. */
+ TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
+ u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
+ );
};
+static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
+ offsetof(struct virtnet_info, rss_hash_key_data));
struct padded_vnet_hdr {
struct virtio_net_hdr_v1_hash hdr;
--
2.43.0On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
>
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>
> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after `struct virtnet_info` (no
> blank line in between).
>
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
>
> struct virtio_net_rss_config_trailer {
> __le16 max_tx_vq; /* 0 2 */
> __u8 hash_key_length; /* 2 1 */
> __u8 hash_key_data[]; /* 3 0 */
>
> /* size: 4, cachelines: 1, members: 3 */
> /* padding: 1 */
> /* last cacheline: 4 bytes */
> };
>
> struct virtnet_info {
> ...
> struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
>
> /* XXX last struct has 1 byte of padding */
>
> u8 rss_hash_key_data[40]; /* 84 40 */
> ...
> /* size: 832, cachelines: 13, members: 48 */
> /* sum members: 801, holes: 8, sum holes: 31 */
> /* paddings: 2, sum paddings: 5 */
> };
>
> After changes, those members are correctly aligned at offset 795:
>
> struct virtnet_info {
> ...
> union {
> struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
> struct {
> unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
> u8 rss_hash_key_data[40]; /* 795 40 */
> }; /* 792 43 */
> }; /* 792 44 */
> ...
> /* size: 840, cachelines: 14, members: 47 */
> /* sum members: 801, holes: 8, sum holes: 35 */
> /* padding: 4 */
> /* paddings: 1, sum paddings: 4 */
> /* last cacheline: 8 bytes */
> };
>
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>
> This should probably include the following tag:
>
> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
>
> but I'd like to hear some feedback, first.
>
> Thanks!
>
I would add:
as a result, the RSS key passed to the device is shifted by 1
byte: the last byte is cut off, and instead a (possibly uninitialized) byte
is added at the beginning.
> drivers/net/virtio_net.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 975bdc5dab84..f4964a18a214 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -425,9 +425,6 @@ struct virtnet_info {
> u16 rss_indir_table_size;
> u32 rss_hash_types_supported;
> u32 rss_hash_types_saved;
> - struct virtio_net_rss_config_hdr *rss_hdr;
> - struct virtio_net_rss_config_trailer rss_trailer;
> - u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
>
> /* Has control virtqueue */
> bool has_cvq;
> @@ -493,7 +490,16 @@ struct virtnet_info {
> struct failover *failover;
>
> u64 device_stats_cap;
> +
> + struct virtio_net_rss_config_hdr *rss_hdr;
> +
> + /* Must be last --ends in a flexible-array member. */
> + TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
> + u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> + );
> };
> +static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
> + offsetof(struct virtnet_info, rss_hash_key_data));
>
> struct padded_vnet_hdr {
> struct virtio_net_hdr_v1_hash hdr;
> --
> 2.43.0
On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
>
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>
> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it (in this case
> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> overlays the trailing members (rss_hash_key_data) onto the FAM
> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> The static_assert() ensures this alignment remains, and it's
> intentionally placed inmediately after `struct virtnet_info` (no
> blank line in between).
>
> Notice that due to tail padding in flexible `struct
> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> offset 84 in struct virtnet_info) are misaligned by one byte. See
> below:
>
> struct virtio_net_rss_config_trailer {
> __le16 max_tx_vq; /* 0 2 */
> __u8 hash_key_length; /* 2 1 */
> __u8 hash_key_data[]; /* 3 0 */
>
> /* size: 4, cachelines: 1, members: 3 */
> /* padding: 1 */
> /* last cacheline: 4 bytes */
> };
>
> struct virtnet_info {
> ...
> struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
>
> /* XXX last struct has 1 byte of padding */
>
> u8 rss_hash_key_data[40]; /* 84 40 */
> ...
> /* size: 832, cachelines: 13, members: 48 */
> /* sum members: 801, holes: 8, sum holes: 31 */
> /* paddings: 2, sum paddings: 5 */
> };
>
> After changes, those members are correctly aligned at offset 795:
>
> struct virtnet_info {
> ...
> union {
> struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
> struct {
> unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
> u8 rss_hash_key_data[40]; /* 795 40 */
> }; /* 792 43 */
> }; /* 792 44 */
> ...
> /* size: 840, cachelines: 14, members: 47 */
> /* sum members: 801, holes: 8, sum holes: 35 */
> /* padding: 4 */
> /* paddings: 1, sum paddings: 4 */
> /* last cacheline: 8 bytes */
> };
>
> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> moved to the end, since it seems those three members should stick
> around together. :)
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>
> This should probably include the following tag:
>
> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
>
> but I'd like to hear some feedback, first.
I tend to agree given that:
On the one hand:
1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
to write random data to .rss_hash_key_data
2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
While
3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
including the contents of .hash_key_data based on the length of
that data provided in .hash_key_length is copied.
It seems to me that step 3 will include 1 byte of uninitialised data
at the start of .hash_key_data. And, correspondingly, truncate
.rss_hash_key_data by one byte.
It's unclear to me what the effect of this - perhaps they key works
regardless. But it doesn't seem intended. And while the result may be
neutral, I do suspect this reduces the quality of the key. And I more
strongly suspect it doesn't have any positive outcome.
So I would lean towards playing it safe and considering this as a bug.
Of course, other's may have better insight as to the actual effect of this.
...
On 9/4/25 11:13, Simon Horman wrote:
> On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>> getting ready to enable it, globally.
>>
>> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>>
>> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>>
>> This helper creates a union between a flexible-array member (FAM)
>> and a set of members that would otherwise follow it (in this case
>> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
>> overlays the trailing members (rss_hash_key_data) onto the FAM
>> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
>> The static_assert() ensures this alignment remains, and it's
>> intentionally placed inmediately after `struct virtnet_info` (no
>> blank line in between).
>>
>> Notice that due to tail padding in flexible `struct
>> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
>> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
>> offset 84 in struct virtnet_info) are misaligned by one byte. See
>> below:
>>
>> struct virtio_net_rss_config_trailer {
>> __le16 max_tx_vq; /* 0 2 */
>> __u8 hash_key_length; /* 2 1 */
>> __u8 hash_key_data[]; /* 3 0 */
>>
>> /* size: 4, cachelines: 1, members: 3 */
>> /* padding: 1 */
>> /* last cacheline: 4 bytes */
>> };
>>
>> struct virtnet_info {
>> ...
>> struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
>>
>> /* XXX last struct has 1 byte of padding */
>>
>> u8 rss_hash_key_data[40]; /* 84 40 */
>> ...
>> /* size: 832, cachelines: 13, members: 48 */
>> /* sum members: 801, holes: 8, sum holes: 31 */
>> /* paddings: 2, sum paddings: 5 */
>> };
>>
>> After changes, those members are correctly aligned at offset 795:
>>
>> struct virtnet_info {
>> ...
>> union {
>> struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
>> struct {
>> unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
>> u8 rss_hash_key_data[40]; /* 795 40 */
>> }; /* 792 43 */
>> }; /* 792 44 */
>> ...
>> /* size: 840, cachelines: 14, members: 47 */
>> /* sum members: 801, holes: 8, sum holes: 35 */
>> /* padding: 4 */
>> /* paddings: 1, sum paddings: 4 */
>> /* last cacheline: 8 bytes */
>> };
>>
>> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
>> moved to the end, since it seems those three members should stick
>> around together. :)
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>
>> This should probably include the following tag:
>>
>> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
>>
>> but I'd like to hear some feedback, first.
>
> I tend to agree given that:
>
> On the one hand:
>
> 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
> to write random data to .rss_hash_key_data
>
> 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
>
> While
>
> 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
> including the contents of .hash_key_data based on the length of
> that data provided in .hash_key_length is copied.
>
> It seems to me that step 3 will include 1 byte of uninitialised data
> at the start of .hash_key_data. And, correspondingly, truncate
> .rss_hash_key_data by one byte.
>
> It's unclear to me what the effect of this - perhaps they key works
> regardless. But it doesn't seem intended. And while the result may be
> neutral, I do suspect this reduces the quality of the key. And I more
> strongly suspect it doesn't have any positive outcome.
>
> So I would lean towards playing it safe and considering this as a bug.
>
> Of course, other's may have better insight as to the actual effect of this.
Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
tags.
Thanks for the feedback!
-Gustavo
On Thu, Sep 04, 2025 at 08:53:31PM +0200, Gustavo A. R. Silva wrote:
>
>
> On 9/4/25 11:13, Simon Horman wrote:
> > On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
> > > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > > getting ready to enable it, globally.
> > >
> > > Use the new TRAILING_OVERLAP() helper to fix the following warning:
> > >
> > > drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > >
> > > This helper creates a union between a flexible-array member (FAM)
> > > and a set of members that would otherwise follow it (in this case
> > > `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
> > > overlays the trailing members (rss_hash_key_data) onto the FAM
> > > (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
> > > The static_assert() ensures this alignment remains, and it's
> > > intentionally placed inmediately after `struct virtnet_info` (no
> > > blank line in between).
> > >
> > > Notice that due to tail padding in flexible `struct
> > > virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
> > > (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
> > > offset 84 in struct virtnet_info) are misaligned by one byte. See
> > > below:
> > >
> > > struct virtio_net_rss_config_trailer {
> > > __le16 max_tx_vq; /* 0 2 */
> > > __u8 hash_key_length; /* 2 1 */
> > > __u8 hash_key_data[]; /* 3 0 */
> > >
> > > /* size: 4, cachelines: 1, members: 3 */
> > > /* padding: 1 */
> > > /* last cacheline: 4 bytes */
> > > };
> > >
> > > struct virtnet_info {
> > > ...
> > > struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
> > >
> > > /* XXX last struct has 1 byte of padding */
> > >
> > > u8 rss_hash_key_data[40]; /* 84 40 */
> > > ...
> > > /* size: 832, cachelines: 13, members: 48 */
> > > /* sum members: 801, holes: 8, sum holes: 31 */
> > > /* paddings: 2, sum paddings: 5 */
> > > };
> > >
> > > After changes, those members are correctly aligned at offset 795:
> > >
> > > struct virtnet_info {
> > > ...
> > > union {
> > > struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
> > > struct {
> > > unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
> > > u8 rss_hash_key_data[40]; /* 795 40 */
> > > }; /* 792 43 */
> > > }; /* 792 44 */
> > > ...
> > > /* size: 840, cachelines: 14, members: 47 */
> > > /* sum members: 801, holes: 8, sum holes: 35 */
> > > /* padding: 4 */
> > > /* paddings: 1, sum paddings: 4 */
> > > /* last cacheline: 8 bytes */
> > > };
> > >
> > > As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
> > > moved to the end, since it seems those three members should stick
> > > around together. :)
> > >
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > > ---
> > >
> > > This should probably include the following tag:
> > >
> > > Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
> > >
> > > but I'd like to hear some feedback, first.
> >
> > I tend to agree given that:
> >
> > On the one hand:
> >
> > 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
> > to write random data to .rss_hash_key_data
> >
> > 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
> >
> > While
> >
> > 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
> > including the contents of .hash_key_data based on the length of
> > that data provided in .hash_key_length is copied.
> >
> > It seems to me that step 3 will include 1 byte of uninitialised data
> > at the start of .hash_key_data. And, correspondingly, truncate
> > .rss_hash_key_data by one byte.
> >
> > It's unclear to me what the effect of this - perhaps they key works
> > regardless. But it doesn't seem intended. And while the result may be
> > neutral, I do suspect this reduces the quality of the key. And I more
> > strongly suspect it doesn't have any positive outcome.
> >
> > So I would lean towards playing it safe and considering this as a bug.
> >
> > Of course, other's may have better insight as to the actual effect of this.
>
> Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
> tags.
>
> Thanks for the feedback!
> -Gustavo
>
>
I agree. It looks like that commit completely broke RSS
configuration. Akihiko do you mind sharing how that was
tested? Maybe help testing the fix? Thanks!
On 9/4/25 23:31, Michael S. Tsirkin wrote:
> On Thu, Sep 04, 2025 at 08:53:31PM +0200, Gustavo A. R. Silva wrote:
>>
>>
>> On 9/4/25 11:13, Simon Horman wrote:
>>> On Wed, Sep 03, 2025 at 09:36:13PM +0200, Gustavo A. R. Silva wrote:
>>>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>>>> getting ready to enable it, globally.
>>>>
>>>> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>>>>
>>>> drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>>>>
>>>> This helper creates a union between a flexible-array member (FAM)
>>>> and a set of members that would otherwise follow it (in this case
>>>> `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
>>>> overlays the trailing members (rss_hash_key_data) onto the FAM
>>>> (hash_key_data) while keeping the FAM and the start of MEMBERS aligned.
>>>> The static_assert() ensures this alignment remains, and it's
>>>> intentionally placed inmediately after `struct virtnet_info` (no
>>>> blank line in between).
>>>>
>>>> Notice that due to tail padding in flexible `struct
>>>> virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data`
>>>> (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at
>>>> offset 84 in struct virtnet_info) are misaligned by one byte. See
>>>> below:
>>>>
>>>> struct virtio_net_rss_config_trailer {
>>>> __le16 max_tx_vq; /* 0 2 */
>>>> __u8 hash_key_length; /* 2 1 */
>>>> __u8 hash_key_data[]; /* 3 0 */
>>>>
>>>> /* size: 4, cachelines: 1, members: 3 */
>>>> /* padding: 1 */
>>>> /* last cacheline: 4 bytes */
>>>> };
>>>>
>>>> struct virtnet_info {
>>>> ...
>>>> struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
>>>>
>>>> /* XXX last struct has 1 byte of padding */
>>>>
>>>> u8 rss_hash_key_data[40]; /* 84 40 */
>>>> ...
>>>> /* size: 832, cachelines: 13, members: 48 */
>>>> /* sum members: 801, holes: 8, sum holes: 31 */
>>>> /* paddings: 2, sum paddings: 5 */
>>>> };
>>>>
>>>> After changes, those members are correctly aligned at offset 795:
>>>>
>>>> struct virtnet_info {
>>>> ...
>>>> union {
>>>> struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */
>>>> struct {
>>>> unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
>>>> u8 rss_hash_key_data[40]; /* 795 40 */
>>>> }; /* 792 43 */
>>>> }; /* 792 44 */
>>>> ...
>>>> /* size: 840, cachelines: 14, members: 47 */
>>>> /* sum members: 801, holes: 8, sum holes: 35 */
>>>> /* padding: 4 */
>>>> /* paddings: 1, sum paddings: 4 */
>>>> /* last cacheline: 8 bytes */
>>>> };
>>>>
>>>> As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also
>>>> moved to the end, since it seems those three members should stick
>>>> around together. :)
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>>>> ---
>>>>
>>>> This should probably include the following tag:
>>>>
>>>> Fixes: ed3100e90d0d ("virtio_net: Use new RSS config structs")
>>>>
>>>> but I'd like to hear some feedback, first.
>>>
>>> I tend to agree given that:
>>>
>>> On the one hand:
>>>
>>> 1) in virtnet_init_default_rss(), netdev_rss_key_fill() is used
>>> to write random data to .rss_hash_key_data
>>>
>>> 2) In virtnet_set_rxfh() key data written to .rss_hash_key_data
>>>
>>> While
>>>
>>> 3) In virtnet_commit_rss_command() virtio_net_rss_config_trailer,
>>> including the contents of .hash_key_data based on the length of
>>> that data provided in .hash_key_length is copied.
>>>
>>> It seems to me that step 3 will include 1 byte of uninitialised data
>>> at the start of .hash_key_data. And, correspondingly, truncate
>>> .rss_hash_key_data by one byte.
>>>
>>> It's unclear to me what the effect of this - perhaps they key works
>>> regardless. But it doesn't seem intended. And while the result may be
>>> neutral, I do suspect this reduces the quality of the key. And I more
>>> strongly suspect it doesn't have any positive outcome.
>>>
>>> So I would lean towards playing it safe and considering this as a bug.
>>>
>>> Of course, other's may have better insight as to the actual effect of this.
>>
>> Yeah, in the meantime I'll prepare v2 with both the 'Fixes' and 'stable'
>> tags.
>>
>> Thanks for the feedback!
>> -Gustavo
>>
>>
>
>
> I agree. It looks like that commit completely broke RSS
> configuration. Akihiko do you mind sharing how that was
> tested? Maybe help testing the fix? Thanks!
I've been waiting for Akihiko's comments on this, but I
guess I'll now go ahead and submit v2 with the Fixes and
stable tags included.
Thanks for the feedback, Michael.
-Gustavo
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