Sorry, forgot to add a subject.
Gabe
On Mon, Jan 23, 2012 at 9:18 PM, Gabe Black <gabebl...@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
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