Re: Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors.

[Sasha Levin <levinsasha928@gmail.com>]

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
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-- 

Sasha.