Re: [PATCH] target/i386/hvf: hide MPX states from XCR0

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/30/24 13:30, Phil Dennis-Jordan wrote:
> On Tue, 29 Oct 2024 at 14:05, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> QEMU does not show availability of MPX in CPUID when running under
>> Hypervisor.framework.  Therefore, in the unlikely chance that the host
>> has MPX enabled, hide those bits from leaf 0xD as well.
> 
> To clarify: is there some kind of issue with MPX in Qemu in general?
> Or is this a consistency effort - normal Macs don't expose this
> feature, so we have no idea if it were to work if someone did manage
> to hack up some frankensteinian host system that somehow does have
> those bits set?

That, and also that real hardware will only show XSTATE_BNDREGS_MASK and 
XSTATE_BNDCSR_MASK if the MPX bit is set in CPUID; which it isn't in 
hvf_get_supported_cpuid().

In fact, for completeness it should also go the other way: if 
XSTATE_YMM_MASK is not set in the result of XGETBV, AVX should be 
hidden.  And if any of OPMASK, ZMM_Hi256 and Hi16_ZMM are not set in the 
result of XGETBV, AVX512F (and AVX10 eventually) should be hidden in 
hvf_get_supported_cpuid().

By the way, could you check if Macs set the PKRU bit of XCR0 (bit 9) 
and/or the OSPKE bit in CPUID (that's bit 4 of CPUID[EAX=7, ECX=0].ECX)?

Thanks,

Paolo

> 
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   target/i386/hvf/x86_cpuid.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target/i386/hvf/x86_cpuid.c b/target/i386/hvf/x86_cpuid.c
>> index e56cd8411ba..4b184767f4a 100644
>> --- a/target/i386/hvf/x86_cpuid.c
>> +++ b/target/i386/hvf/x86_cpuid.c
>> @@ -110,9 +110,9 @@ uint32_t hvf_get_supported_cpuid(uint32_t func, uint32_t idx,
>>           if (idx == 0) {
>>               uint64_t host_xcr0;
>>               if (xgetbv(ecx, 0, &host_xcr0)) {
>> +                /* Only show xcr0 bits corresponding to usable features.  */
>>                   uint64_t supp_xcr0 = host_xcr0 & (XSTATE_FP_MASK |
>>                                     XSTATE_SSE_MASK | XSTATE_YMM_MASK |
>> -                                  XSTATE_BNDREGS_MASK | XSTATE_BNDCSR_MASK |
>>                                     XSTATE_OPMASK_MASK | XSTATE_ZMM_Hi256_MASK |
>>                                     XSTATE_Hi16_ZMM_MASK);
>>                   eax &= supp_xcr0;
>> --
>> 2.47.0
>>
>>
> 
>