Re: [PATCH 3/3] x86: Use CpuidUserDis if an AMD HVM guest toggles CPUID faulting

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.05.2023 12:05, Andrew Cooper wrote:
> On 08/05/2023 2:18 pm, Jan Beulich wrote:
>> On 05.05.2023 19:57, Alejandro Vallejo wrote:
>>> This is in order to aid guests of AMD hardware that we have exposed
>>> CPUID faulting to. If they try to modify the Intel MSR that enables
>>> the feature, trigger levelling so AMD's version of it (CpuidUserDis)
>>> is used instead.
>>>
>>> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
>>> ---
>>>  xen/arch/x86/msr.c | 9 ++++++++-
>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>> Don't you also need to update cpu-policy.c:calculate_host_policy()
>> for the guest to actually know it can use the functionality? Which
>> in turn would appear to require some form of adjustment to
>> lib/x86/policy.c:x86_cpu_policies_are_compatible().
> 
> I asked Alejandro to do it like this.
> 
> Advertising this to guests requires plumbing another MSR into the
> infrastructure which isn't quite set up properly let, and is in flux
> from my work.

Maybe there was some misunderstanding here, as I realize only now. I
wasn't asking to expose the AMD feature, but instead I was after

    /* 0x000000ce  MSR_INTEL_PLATFORM_INFO */
    /* probe_cpuid_faulting() sanity checks presence of MISC_FEATURES_ENABLES */
    p->platform_info.cpuid_faulting = cpu_has_cpuid_faulting;

wanting to be extended by "|| boot_cpu_has(X86_FEATURE_CPUID_USER_DIS)".
That, afaict, has no connection to plumbing yet another MSR.

Jan