Re: [Xen-devel] [xen-unstable test] 33399: regressions - FAIL

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.15 at 15:14, <JBeulich@xxxxxxxx> wrote:
> But I think I made a wrong assumption above regarding the
> guest size: test-amd64-i386-xl-win7-amd64 produces a 64-bit
> guest with a 32-bit tool stack, so the crucial part of all the
> tests failing is not the guest's bitness, but tool stack's. So I'll
> next look into which of the three feature flags might be off
> when inspected from a 32-bit Dom0, as I now suspect that the
> guest simply doesn't get its CPUID bits correctly set up by a
> 32-bit Dom0 (i.e. the patch might just have uncovered a latent
> bug).

And there you go: The hypervisor deliberately clears the
syscall feature flag for 32-bit PV guests on non-AMD CPUs, and
hardware appears to do so too when CPUID gets executed from
a non-64-bit CS (i.e. no matter whether you execute raw or
"Xen-ified" CPUID there, you won't see that flag set). Yet 64-bit
guests won't be bothered to check whether the flag is enabled,
as x86-64 requires the feature to be there.

But I think even if this could be taken care of in the tool stack,
it's better to have hvm_cpuid() mimic that behavior, i.e. force
FEATURE_SYSCALL on when hvm_guest_x86_mode() == 8.

Jan


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