* Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors.
@ 2012-01-24 5:20 Gabe Black
2012-01-24 7:40 ` Sasha Levin
0 siblings, 1 reply; 3+ messages in thread
From: Gabe Black @ 2012-01-24 5:20 UTC (permalink / raw)
To: kvm; +Cc: Ronald Minnich
Sorry, forgot to add a subject.
Gabe
On Mon, Jan 23, 2012 at 9:18 PM, Gabe Black <gabeblack@google.com> wrote:
> Hi, I think I've tracked down the bug that causes
> "KVM_GET_SUPPORTED_CPUID failed: Argument list too long" errors when
> using the kvm tool. Basically, this (possibly squished) code seems to
> be to blame:
>
> case 0xd: {
> int i;
>
> entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> for (i = 1; *nent < maxnent && i < 64; ++i) {
> if (entry[i].eax == 0)
> continue;
> do_cpuid_1_ent(&entry[i], function, i);
> entry[i].flags |=
> KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> ++*nent;
> }
> break;
> }
>
> You can see there's a check whether entry[i].eax is 0, but it isn't
> until the next line that entry[i] is actually filled in. That means
> that whether or not an entry is filled in for the 0xd function is
> essentially random, and that can lead to the loss of valid entries. It
> also means that nent may be incremented too often, and since all 64
> entries are iterated over, that can fill up the available storage and
> cause that error.
>
> I tested my theory by commenting out the if (100% failure rate) and
> moving it after do_cpuid_1_ent (100% success rate). Since this is a
> non-deterministic failure that isn't really conclusive, but I'm fairly
> confident my fix is correct. I don't know exactly what your procedure
> is for submitting patches, but one is attached.
>
> Gabe
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors.
2012-01-24 5:20 Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors Gabe Black
@ 2012-01-24 7:40 ` Sasha Levin
2012-01-24 21:53 ` Gabe Black
0 siblings, 1 reply; 3+ messages in thread
From: Sasha Levin @ 2012-01-24 7:40 UTC (permalink / raw)
To: Gabe Black; +Cc: kvm, Ronald Minnich
The GET_SUPPORTED_CPUID bug has been fixed and shouldn't be happening
from v3.2 onwards.
Do you still see the issue in older versions?
On Mon, 2012-01-23 at 21:20 -0800, Gabe Black wrote:
> Sorry, forgot to add a subject.
>
> Gabe
>
> On Mon, Jan 23, 2012 at 9:18 PM, Gabe Black <gabeblack@google.com> wrote:
> > Hi, I think I've tracked down the bug that causes
> > "KVM_GET_SUPPORTED_CPUID failed: Argument list too long" errors when
> > using the kvm tool. Basically, this (possibly squished) code seems to
> > be to blame:
> >
> > case 0xd: {
> > int i;
> >
> > entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > for (i = 1; *nent < maxnent && i < 64; ++i) {
> > if (entry[i].eax == 0)
> > continue;
> > do_cpuid_1_ent(&entry[i], function, i);
> > entry[i].flags |=
> > KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > ++*nent;
> > }
> > break;
> > }
> >
> > You can see there's a check whether entry[i].eax is 0, but it isn't
> > until the next line that entry[i] is actually filled in. That means
> > that whether or not an entry is filled in for the 0xd function is
> > essentially random, and that can lead to the loss of valid entries. It
> > also means that nent may be incremented too often, and since all 64
> > entries are iterated over, that can fill up the available storage and
> > cause that error.
> >
> > I tested my theory by commenting out the if (100% failure rate) and
> > moving it after do_cpuid_1_ent (100% success rate). Since this is a
> > non-deterministic failure that isn't really conclusive, but I'm fairly
> > confident my fix is correct. I don't know exactly what your procedure
> > is for submitting patches, but one is attached.
> >
> > Gabe
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
Sasha.
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors.
2012-01-24 7:40 ` Sasha Levin
@ 2012-01-24 21:53 ` Gabe Black
0 siblings, 0 replies; 3+ messages in thread
From: Gabe Black @ 2012-01-24 21:53 UTC (permalink / raw)
To: Sasha Levin; +Cc: kvm, Ronald Minnich
Yes, I saw the problem with 3.0.13. Thanks!
Gabe
On Mon, Jan 23, 2012 at 11:40 PM, Sasha Levin <levinsasha928@gmail.com> wrote:
> The GET_SUPPORTED_CPUID bug has been fixed and shouldn't be happening
> from v3.2 onwards.
>
> Do you still see the issue in older versions?
>
> On Mon, 2012-01-23 at 21:20 -0800, Gabe Black wrote:
>> Sorry, forgot to add a subject.
>>
>> Gabe
>>
>> On Mon, Jan 23, 2012 at 9:18 PM, Gabe Black <gabeblack@google.com> wrote:
>> > Hi, I think I've tracked down the bug that causes
>> > "KVM_GET_SUPPORTED_CPUID failed: Argument list too long" errors when
>> > using the kvm tool. Basically, this (possibly squished) code seems to
>> > be to blame:
>> >
>> > case 0xd: {
>> > int i;
>> >
>> > entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> > for (i = 1; *nent < maxnent && i < 64; ++i) {
>> > if (entry[i].eax == 0)
>> > continue;
>> > do_cpuid_1_ent(&entry[i], function, i);
>> > entry[i].flags |=
>> > KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> > ++*nent;
>> > }
>> > break;
>> > }
>> >
>> > You can see there's a check whether entry[i].eax is 0, but it isn't
>> > until the next line that entry[i] is actually filled in. That means
>> > that whether or not an entry is filled in for the 0xd function is
>> > essentially random, and that can lead to the loss of valid entries. It
>> > also means that nent may be incremented too often, and since all 64
>> > entries are iterated over, that can fill up the available storage and
>> > cause that error.
>> >
>> > I tested my theory by commenting out the if (100% failure rate) and
>> > moving it after do_cpuid_1_ent (100% success rate). Since this is a
>> > non-deterministic failure that isn't really conclusive, but I'm fairly
>> > confident my fix is correct. I don't know exactly what your procedure
>> > is for submitting patches, but one is attached.
>> >
>> > Gabe
>> --
>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
>
> --
>
> Sasha.
>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2012-01-24 5:20 Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors Gabe Black
2012-01-24 7:40 ` Sasha Levin
2012-01-24 21:53 ` Gabe Black
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