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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.15 at 16:06, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/01/15 14:53, Jan Beulich wrote:
>>>>> 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.
> 
> As AMD had supported syscall in 32bit systems for a long time, I presume
> it is only Intel where the feature bit in cpuid changes depending on cs.L

Sure.

Jan


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