This commit solves the issue with userfault_fd WP feature that
background snapshot is based on. For any never poluated or discarded
memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating
PTE for that page, thereby loosing WP setting for it.
So we need to pre-fault pages for each RAM block to be protected
before making a userfault_fd wr-protect ioctl().
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
---
migration/migration.c | 6 +++++
migration/ram.c | 51 +++++++++++++++++++++++++++++++++++++++++++
migration/ram.h | 1 +
3 files changed, 58 insertions(+)
diff --git a/migration/migration.c b/migration/migration.c
index 656d6249a6..496e88cbda 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3872,6 +3872,12 @@ static void *bg_migration_thread(void *opaque)
update_iteration_initial_status(s);
+ /*
+ * Prepare for tracking memory writes with UFFD-WP - populate
+ * RAM pages before protecting.
+ */
+ ram_write_tracking_prepare();
+
qemu_savevm_state_header(s->to_dst_file);
qemu_savevm_state_setup(s->to_dst_file);
diff --git a/migration/ram.c b/migration/ram.c
index 52537f14ac..825eb80030 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1560,6 +1560,57 @@ out:
return ret;
}
+/*
+ * ram_block_populate_pages: populate memory in the RAM block by reading
+ * an integer from the beginning of each page.
+ *
+ * Since it's solely used for userfault_fd WP feature, here we just
+ * hardcode page size to TARGET_PAGE_SIZE.
+ *
+ * @bs: RAM block to populate
+ */
+volatile int ram_block_populate_pages__tmp;
+static void ram_block_populate_pages(RAMBlock *bs)
+{
+ ram_addr_t offset = 0;
+ int tmp = 0;
+
+ for (char *ptr = (char *) bs->host; offset < bs->used_length;
+ ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
+ /* Try to do it without memory writes */
+ tmp += *(volatile int *) ptr;
+ }
+ /* Create dependency on 'extern volatile int' to avoid optimizing out */
+ ram_block_populate_pages__tmp += tmp;
+}
+
+/*
+ * ram_write_tracking_prepare: prepare for UFFD-WP memory tracking
+ */
+void ram_write_tracking_prepare(void)
+{
+ RAMBlock *bs;
+
+ RCU_READ_LOCK_GUARD();
+
+ RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+ /* Nothing to do with read-only and MMIO-writable regions */
+ if (bs->mr->readonly || bs->mr->rom_device) {
+ continue;
+ }
+
+ /*
+ * Populate pages of the RAM block before enabling userfault_fd
+ * write protection.
+ *
+ * This stage is required since ioctl(UFFDIO_WRITEPROTECT) with
+ * UFFDIO_WRITEPROTECT_MODE_WP mode setting would silently skip
+ * pages with pte_none() entries in page table.
+ */
+ ram_block_populate_pages(bs);
+ }
+}
+
/*
* ram_write_tracking_start: start UFFD-WP memory tracking
*
diff --git a/migration/ram.h b/migration/ram.h
index 6378bb3ebc..4833e9fd5b 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -82,6 +82,7 @@ void colo_incoming_start_dirty_log(void);
/* Background snapshot */
bool ram_write_tracking_available(void);
bool ram_write_tracking_compatible(void);
+void ram_write_tracking_prepare(void);
int ram_write_tracking_start(void);
void ram_write_tracking_stop(void);
--
2.25.1
> +/*
> + * ram_block_populate_pages: populate memory in the RAM block by reading
> + * an integer from the beginning of each page.
> + *
> + * Since it's solely used for userfault_fd WP feature, here we just
> + * hardcode page size to TARGET_PAGE_SIZE.
> + *
> + * @bs: RAM block to populate
> + */
> +volatile int ram_block_populate_pages__tmp;
> +static void ram_block_populate_pages(RAMBlock *bs)
> +{
> + ram_addr_t offset = 0;
> + int tmp = 0;
> +
> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
> + /* Try to do it without memory writes */
> + tmp += *(volatile int *) ptr;
> + }
The following is slightly simpler and doesn't rely on volatile semantics [1].
Should work on any arch I guess.
static void ram_block_populate_pages(RAMBlock *bs)
{
char *ptr = (char *) bs->host;
ram_addr_t offset;
for (offset = 0; offset < bs->used_length;
offset += qemu_real_host_page_size) {
char tmp = *(volatile char *)(ptr + offset)
/* Don't optimize the read out. */
asm volatile ("" : "+r" (tmp));
}
Compiles to
for (offset = 0; offset < bs->used_length;
316d: 48 8b 4b 30 mov 0x30(%rbx),%rcx
char *ptr = (char *) bs->host;
3171: 48 8b 73 18 mov 0x18(%rbx),%rsi
for (offset = 0; offset < bs->used_length;
3175: 48 85 c9 test %rcx,%rcx
3178: 74 ce je 3148 <ram_write_tracking_prepare+0x58>
offset += qemu_real_host_page_size) {
317a: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 3181 <ram_write_tracking_prepare+0x91>
3181: 48 8b 38 mov (%rax),%rdi
3184: 0f 1f 40 00 nopl 0x0(%rax)
char tmp = *(volatile char *)(ptr + offset);
3188: 48 8d 04 16 lea (%rsi,%rdx,1),%rax
318c: 0f b6 00 movzbl (%rax),%eax
offset += qemu_real_host_page_size) {
318f: 48 01 fa add %rdi,%rdx
for (offset = 0; offset < bs->used_length;
3192: 48 39 ca cmp %rcx,%rdx
3195: 72 f1 jb 3188 <ram_write_tracking_prepare+0x98>
[1] https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/
I'll send patches soon to take care of virtio-mem via RamDiscardManager -
to skip populating the parts that are supposed to remain discarded and not migrated.
Unfortunately, the RamDiscardManager patches are still stuck waiting for
acks ... and now we're in soft-freeze.
--
Thanks,
David / dhildenb
On 19.03.21 10:28, David Hildenbrand wrote:
>> +/*
>> + * ram_block_populate_pages: populate memory in the RAM block by reading
>> + * an integer from the beginning of each page.
>> + *
>> + * Since it's solely used for userfault_fd WP feature, here we just
>> + * hardcode page size to TARGET_PAGE_SIZE.
>> + *
>> + * @bs: RAM block to populate
>> + */
>> +volatile int ram_block_populate_pages__tmp;
>> +static void ram_block_populate_pages(RAMBlock *bs)
>> +{
>> + ram_addr_t offset = 0;
>> + int tmp = 0;
>> +
>> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
>> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
>
> You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
>
>> + /* Try to do it without memory writes */
>> + tmp += *(volatile int *) ptr;
>> + }
>
>
> The following is slightly simpler and doesn't rely on volatile semantics [1].
> Should work on any arch I guess.
>
> static void ram_block_populate_pages(RAMBlock *bs)
> {
> char *ptr = (char *) bs->host;
> ram_addr_t offset;
>
> for (offset = 0; offset < bs->used_length;
> offset += qemu_real_host_page_size) {
> char tmp = *(volatile char *)(ptr + offset)
I wanted to do a "= *(ptr + offset)" here.
>
> /* Don't optimize the read out. */
> asm volatile ("" : "+r" (tmp));
So this is the only volatile thing that the compiler must guarantee to
not optimize away.
--
Thanks,
David / dhildenb
On 19.03.2021 12:32, David Hildenbrand wrote:
> On 19.03.21 10:28, David Hildenbrand wrote:
>>> +/*
>>> + * ram_block_populate_pages: populate memory in the RAM block by
>>> reading
>>> + * an integer from the beginning of each page.
>>> + *
>>> + * Since it's solely used for userfault_fd WP feature, here we just
>>> + * hardcode page size to TARGET_PAGE_SIZE.
>>> + *
>>> + * @bs: RAM block to populate
>>> + */
>>> +volatile int ram_block_populate_pages__tmp;
>>> +static void ram_block_populate_pages(RAMBlock *bs)
>>> +{
>>> + ram_addr_t offset = 0;
>>> + int tmp = 0;
>>> +
>>> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
>>> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
>>
>> You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
>>
>>> + /* Try to do it without memory writes */
>>> + tmp += *(volatile int *) ptr;
>>> + }
>>
>>
>> The following is slightly simpler and doesn't rely on volatile
>> semantics [1].
>> Should work on any arch I guess.
>>
>> static void ram_block_populate_pages(RAMBlock *bs)
>> {
>> char *ptr = (char *) bs->host;
>> ram_addr_t offset;
>>
>> for (offset = 0; offset < bs->used_length;
>> offset += qemu_real_host_page_size) {
>> char tmp = *(volatile char *)(ptr + offset)
>
> I wanted to do a "= *(ptr + offset)" here.
>
Yep
>>
>> /* Don't optimize the read out. */
>> asm volatile ("" : "+r" (tmp));
>
> So this is the only volatile thing that the compiler must guarantee to
> not optimize away.
>
>
--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH +7-903-247-6397
virtuzzo.com
On 19.03.2021 12:28, David Hildenbrand wrote:
>> +/*
>> + * ram_block_populate_pages: populate memory in the RAM block by
>> reading
>> + * an integer from the beginning of each page.
>> + *
>> + * Since it's solely used for userfault_fd WP feature, here we just
>> + * hardcode page size to TARGET_PAGE_SIZE.
>> + *
>> + * @bs: RAM block to populate
>> + */
>> +volatile int ram_block_populate_pages__tmp;
>> +static void ram_block_populate_pages(RAMBlock *bs)
>> +{
>> + ram_addr_t offset = 0;
>> + int tmp = 0;
>> +
>> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
>> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
>
> You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
>
Ok.
>> + /* Try to do it without memory writes */
>> + tmp += *(volatile int *) ptr;
>> + }
>
>
> The following is slightly simpler and doesn't rely on volatile
> semantics [1].
> Should work on any arch I guess.
>
> static void ram_block_populate_pages(RAMBlock *bs)
> {
> char *ptr = (char *) bs->host;
> ram_addr_t offset;
>
> for (offset = 0; offset < bs->used_length;
> offset += qemu_real_host_page_size) {
> char tmp = *(volatile char *)(ptr + offset)
>
> /* Don't optimize the read out. */
> asm volatile ("" : "+r" (tmp));
> }
>
Thanks, good option, I'll change the code.
> Compiles to
>
> for (offset = 0; offset < bs->used_length;
> 316d: 48 8b 4b 30 mov 0x30(%rbx),%rcx
> char *ptr = (char *) bs->host;
> 3171: 48 8b 73 18 mov 0x18(%rbx),%rsi
> for (offset = 0; offset < bs->used_length;
> 3175: 48 85 c9 test %rcx,%rcx
> 3178: 74 ce je 3148
> <ram_write_tracking_prepare+0x58>
> offset += qemu_real_host_page_size) {
> 317a: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax #
> 3181 <ram_write_tracking_prepare+0x91>
> 3181: 48 8b 38 mov (%rax),%rdi
> 3184: 0f 1f 40 00 nopl 0x0(%rax)
> char tmp = *(volatile char *)(ptr + offset);
> 3188: 48 8d 04 16 lea (%rsi,%rdx,1),%rax
> 318c: 0f b6 00 movzbl (%rax),%eax
> offset += qemu_real_host_page_size) {
> 318f: 48 01 fa add %rdi,%rdx
> for (offset = 0; offset < bs->used_length;
> 3192: 48 39 ca cmp %rcx,%rdx
> 3195: 72 f1 jb 3188
> <ram_write_tracking_prepare+0x98>
>
>
> [1]
> https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/
>
>
> I'll send patches soon to take care of virtio-mem via RamDiscardManager -
> to skip populating the parts that are supposed to remain discarded and
> not migrated.
> Unfortunately, the RamDiscardManager patches are still stuck waiting for
> acks ... and now we're in soft-freeze.
>
RamDiscardManager patches - do they also modify migration code?
I mean which part is responsible of not migrating discarded ranges.
--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH +7-903-247-6397
virtuzzo.com
On 19.03.21 12:05, Andrey Gruzdev wrote:
> On 19.03.2021 12:28, David Hildenbrand wrote:
>>> +/*
>>> + * ram_block_populate_pages: populate memory in the RAM block by
>>> reading
>>> + * an integer from the beginning of each page.
>>> + *
>>> + * Since it's solely used for userfault_fd WP feature, here we just
>>> + * hardcode page size to TARGET_PAGE_SIZE.
>>> + *
>>> + * @bs: RAM block to populate
>>> + */
>>> +volatile int ram_block_populate_pages__tmp;
>>> +static void ram_block_populate_pages(RAMBlock *bs)
>>> +{
>>> + ram_addr_t offset = 0;
>>> + int tmp = 0;
>>> +
>>> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
>>> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
>>
>> You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
>>
> Ok.
>>> + /* Try to do it without memory writes */
>>> + tmp += *(volatile int *) ptr;
>>> + }
>>
>>
>> The following is slightly simpler and doesn't rely on volatile
>> semantics [1].
>> Should work on any arch I guess.
>>
>> static void ram_block_populate_pages(RAMBlock *bs)
>> {
>> char *ptr = (char *) bs->host;
>> ram_addr_t offset;
>>
>> for (offset = 0; offset < bs->used_length;
>> offset += qemu_real_host_page_size) {
>> char tmp = *(volatile char *)(ptr + offset)
>>
>> /* Don't optimize the read out. */
>> asm volatile ("" : "+r" (tmp));
>> }
>>
> Thanks, good option, I'll change the code.
>
>> Compiles to
>>
>> for (offset = 0; offset < bs->used_length;
>> 316d: 48 8b 4b 30 mov 0x30(%rbx),%rcx
>> char *ptr = (char *) bs->host;
>> 3171: 48 8b 73 18 mov 0x18(%rbx),%rsi
>> for (offset = 0; offset < bs->used_length;
>> 3175: 48 85 c9 test %rcx,%rcx
>> 3178: 74 ce je 3148
>> <ram_write_tracking_prepare+0x58>
>> offset += qemu_real_host_page_size) {
>> 317a: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax #
>> 3181 <ram_write_tracking_prepare+0x91>
>> 3181: 48 8b 38 mov (%rax),%rdi
>> 3184: 0f 1f 40 00 nopl 0x0(%rax)
>> char tmp = *(volatile char *)(ptr + offset);
>> 3188: 48 8d 04 16 lea (%rsi,%rdx,1),%rax
>> 318c: 0f b6 00 movzbl (%rax),%eax
>> offset += qemu_real_host_page_size) {
>> 318f: 48 01 fa add %rdi,%rdx
>> for (offset = 0; offset < bs->used_length;
>> 3192: 48 39 ca cmp %rcx,%rdx
>> 3195: 72 f1 jb 3188
>> <ram_write_tracking_prepare+0x98>
>>
>>
>> [1]
>> https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/
>>
>>
>> I'll send patches soon to take care of virtio-mem via RamDiscardManager -
>> to skip populating the parts that are supposed to remain discarded and
>> not migrated.
>> Unfortunately, the RamDiscardManager patches are still stuck waiting for
>> acks ... and now we're in soft-freeze.
>>
> RamDiscardManager patches - do they also modify migration code?
> I mean which part is responsible of not migrating discarded ranges.
I haven't shared relevant patches yet that touch migration code. I'm
planning on doing that once the generic RamDiscardManager has all
relevant acks. I'll put you on cc.
--
Thanks,
David / dhildenb
On 19.03.2021 14:27, David Hildenbrand wrote:
> On 19.03.21 12:05, Andrey Gruzdev wrote:
>> On 19.03.2021 12:28, David Hildenbrand wrote:
>>>> +/*
>>>> + * ram_block_populate_pages: populate memory in the RAM block by
>>>> reading
>>>> + * an integer from the beginning of each page.
>>>> + *
>>>> + * Since it's solely used for userfault_fd WP feature, here we just
>>>> + * hardcode page size to TARGET_PAGE_SIZE.
>>>> + *
>>>> + * @bs: RAM block to populate
>>>> + */
>>>> +volatile int ram_block_populate_pages__tmp;
>>>> +static void ram_block_populate_pages(RAMBlock *bs)
>>>> +{
>>>> + ram_addr_t offset = 0;
>>>> + int tmp = 0;
>>>> +
>>>> + for (char *ptr = (char *) bs->host; offset < bs->used_length;
>>>> + ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
>>>
>>> You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
>>>
>> Ok.
>>>> + /* Try to do it without memory writes */
>>>> + tmp += *(volatile int *) ptr;
>>>> + }
>>>
>>>
>>> The following is slightly simpler and doesn't rely on volatile
>>> semantics [1].
>>> Should work on any arch I guess.
>>>
>>> static void ram_block_populate_pages(RAMBlock *bs)
>>> {
>>> char *ptr = (char *) bs->host;
>>> ram_addr_t offset;
>>>
>>> for (offset = 0; offset < bs->used_length;
>>> offset += qemu_real_host_page_size) {
>>> char tmp = *(volatile char *)(ptr + offset)
>>>
>>> /* Don't optimize the read out. */
>>> asm volatile ("" : "+r" (tmp));
>>> }
>>>
>> Thanks, good option, I'll change the code.
>>
>>> Compiles to
>>>
>>> for (offset = 0; offset < bs->used_length;
>>> 316d: 48 8b 4b 30 mov 0x30(%rbx),%rcx
>>> char *ptr = (char *) bs->host;
>>> 3171: 48 8b 73 18 mov 0x18(%rbx),%rsi
>>> for (offset = 0; offset < bs->used_length;
>>> 3175: 48 85 c9 test %rcx,%rcx
>>> 3178: 74 ce je 3148
>>> <ram_write_tracking_prepare+0x58>
>>> offset += qemu_real_host_page_size) {
>>> 317a: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax #
>>> 3181 <ram_write_tracking_prepare+0x91>
>>> 3181: 48 8b 38 mov (%rax),%rdi
>>> 3184: 0f 1f 40 00 nopl 0x0(%rax)
>>> char tmp = *(volatile char *)(ptr + offset);
>>> 3188: 48 8d 04 16 lea (%rsi,%rdx,1),%rax
>>> 318c: 0f b6 00 movzbl (%rax),%eax
>>> offset += qemu_real_host_page_size) {
>>> 318f: 48 01 fa add %rdi,%rdx
>>> for (offset = 0; offset < bs->used_length;
>>> 3192: 48 39 ca cmp %rcx,%rdx
>>> 3195: 72 f1 jb 3188
>>> <ram_write_tracking_prepare+0x98>
>>>
>>>
>>> [1]
>>> https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/
>>>
>>>
>>>
>>> I'll send patches soon to take care of virtio-mem via
>>> RamDiscardManager -
>>> to skip populating the parts that are supposed to remain discarded and
>>> not migrated.
>>> Unfortunately, the RamDiscardManager patches are still stuck waiting
>>> for
>>> acks ... and now we're in soft-freeze.
>>>
>> RamDiscardManager patches - do they also modify migration code?
>> I mean which part is responsible of not migrating discarded ranges.
>
> I haven't shared relevant patches yet that touch migration code. I'm
> planning on doing that once the generic RamDiscardManager has all
> relevant acks. I'll put you on cc.
>
Got it, thanks.
--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH +7-903-247-6397
virtuzzo.com
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