[Xen-devel] [PATCH v2] psr: fix bug which may cause crash

Yi Sun posted 1 patch 1 week ago
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/xen tags/patchew/1574936284-5139-1-git-send-email-yi.y.sun@linux.intel.com
xen/arch/x86/psr.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

[Xen-devel] [PATCH v2] psr: fix bug which may cause crash

Posted by Yi Sun 1 week ago
During test, we found a crash on Xen with below trace.
(XEN) Xen call trace:
(XEN)    [<ffff82d0802a065a>] R psr.c#l3_cdp_write_msr+0x1e/0x22
(XEN)    [<ffff82d0802a0858>] F psr.c#do_write_psr_msrs+0x6d/0x109
(XEN)    [<ffff82d08023e000>] F smp_call_function_interrupt+0x5a/0xac
(XEN)    [<ffff82d0802a2b89>] F call_function_interrupt+0x20/0x34
(XEN)    [<ffff82d080282c64>] F do_IRQ+0x175/0x6ae
(XEN)    [<ffff82d08038b8ba>] F common_interrupt+0x10a/0x120
(XEN)    [<ffff82d0802ec616>] F cpu_idle.c#acpi_idle_do_entry+0x9d/0xb1
(XEN)    [<ffff82d0802ecc01>] F cpu_idle.c#acpi_processor_idle+0x41d/0x626
(XEN)    [<ffff82d08027353b>] F domain.c#idle_loop+0xa5/0xa7
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 20:
(XEN) GENERAL PROTECTION FAULT
(XEN) [error_code=0000]
(XEN) ****************************************

The bug happens when CDP and MBA co-exist and MBA COS_MAX is bigger
than CDP COS_MAX. E.g. MBA has 8 COS registers but CDP only have 6.
When setting MBA throttling value for the 7th guest, the value array
would be:
    +------------------+------------------+--------------+
    | Data default val | Code default val | MBA throttle |
    +------------------+------------------+--------------+

Then, COS id 7 will be selected for writting the values. We should
avoid writting CDP data/code valules to COS id 7 MSR because it
exceeds the CDP COS_MAX.

Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
---
 xen/arch/x86/psr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/psr.c b/xen/arch/x86/psr.c
index 5866a26..ecca5b4 100644
--- a/xen/arch/x86/psr.c
+++ b/xen/arch/x86/psr.c
@@ -1271,7 +1271,8 @@ static void do_write_psr_msrs(void *data)
 
         for ( j = 0; j < cos_num; j++, index++ )
         {
-            if ( feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
+            if ( cos <= feat->cos_max &&
+                 feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
             {
                 feat->cos_reg_val[cos * cos_num + j] = info->val[index];
                 props->write_msr(cos, info->val[index], props->type[j]);
-- 
1.9.1


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Re: [Xen-devel] [PATCH v2] psr: fix bug which may cause crash

Posted by Jan Beulich 1 week ago
On 28.11.2019 11:18, Yi Sun wrote:
> --- a/xen/arch/x86/psr.c
> +++ b/xen/arch/x86/psr.c
> @@ -1271,7 +1271,8 @@ static void do_write_psr_msrs(void *data)
>  
>          for ( j = 0; j < cos_num; j++, index++ )
>          {
> -            if ( feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
> +            if ( cos <= feat->cos_max &&
> +                 feat->cos_reg_val[cos * cos_num + j] != info->val[index] )

The description is indeed much better now, thanks. However,
as indicated in reply to v1, this extra (and at the first glance
unmotivated) bounds check wants to be accompanied by a brief but
precise comment. Furthermore with the loop bounded by a local
variable, why not

        cos_num = min(props->cos_num, feat->cos_max + 1);

a few lines up from here (again suitable commented)?

Jan

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Re: [Xen-devel] [PATCH v2] psr: fix bug which may cause crash

Posted by Yi Sun 1 week ago
On 19-11-28 12:25:44, Jan Beulich wrote:
> On 28.11.2019 11:18, Yi Sun wrote:
> > --- a/xen/arch/x86/psr.c
> > +++ b/xen/arch/x86/psr.c
> > @@ -1271,7 +1271,8 @@ static void do_write_psr_msrs(void *data)
> >  
> >          for ( j = 0; j < cos_num; j++, index++ )
> >          {
> > -            if ( feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
> > +            if ( cos <= feat->cos_max &&
> > +                 feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
> 
> The description is indeed much better now, thanks. However,
> as indicated in reply to v1, this extra (and at the first glance
> unmotivated) bounds check wants to be accompanied by a brief but

I will add the comment.

> precise comment. Furthermore with the loop bounded by a local
> variable, why not
> 
>         cos_num = min(props->cos_num, feat->cos_max + 1);
> 
> a few lines up from here (again suitable commented)?
>
cos_num is a different thing with the number of COS registers. The
meaning of it is "COS registers number that feature uses for one COS
ID". E.g. MBA/CAT cos_num is always 1. But CDP cos_num is 2 because it
uses 2 COS registers for one COS ID.
 
> Jan

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Re: [Xen-devel] [PATCH v2] psr: fix bug which may cause crash

Posted by Jan Beulich 1 week ago
On 29.11.2019 02:41, Yi Sun wrote:
> On 19-11-28 12:25:44, Jan Beulich wrote:
>> On 28.11.2019 11:18, Yi Sun wrote:
>>> --- a/xen/arch/x86/psr.c
>>> +++ b/xen/arch/x86/psr.c
>>> @@ -1271,7 +1271,8 @@ static void do_write_psr_msrs(void *data)
>>>  
>>>          for ( j = 0; j < cos_num; j++, index++ )
>>>          {
>>> -            if ( feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
>>> +            if ( cos <= feat->cos_max &&
>>> +                 feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
>>
>> The description is indeed much better now, thanks. However,
>> as indicated in reply to v1, this extra (and at the first glance
>> unmotivated) bounds check wants to be accompanied by a brief but
> 
> I will add the comment.
> 
>> precise comment. Furthermore with the loop bounded by a local
>> variable, why not
>>
>>         cos_num = min(props->cos_num, feat->cos_max + 1);
>>
>> a few lines up from here (again suitable commented)?
>>
> cos_num is a different thing with the number of COS registers. The
> meaning of it is "COS registers number that feature uses for one COS
> ID". E.g. MBA/CAT cos_num is always 1. But CDP cos_num is 2 because it
> uses 2 COS registers for one COS ID.

Oh, right, I got confused. The cos <= feat->cos_max is entirely
loop invariant. Hence the loop shouldn't even be entered, i.e.
you want to add a conditional ahead of it.

Jan

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