Re: [Xen-devel] [PATCH] x86: ignore guest microcode loading attempts

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/03/18 10:40, Jan Beulich wrote:
>
>>> @@ -200,6 +202,16 @@ int guest_wrmsr(struct vcpu *v, uint32_t
>>>          /* Read-only */
>>>          goto gp_fault;
>>>  
>>> +    case MSR_AMD_PATCHLOADER:
>>> +        if ( d->arch.cpuid->x86_vendor != X86_VENDOR_AMD )
>>> +            goto gp_fault;
>>> +        break;
>>> +
>>> +    case MSR_IA32_UCODE_WRITE:
>>> +        if ( d->arch.cpuid->x86_vendor != X86_VENDOR_INTEL )
>>> +            goto gp_fault;
>> Can we leave a note here that Windows at least on some hardware loads
>> microcode before setting up an IDT/GDT, and will triple fault if we hand
>> it back #GP.
> Will do.
>
>>  Ignoring the write means windows will see the same
>> microcode version after the load attempt, and conclude that it didn't
>> succeed?
> That's what I imply. After all things have worked before, where
> we also silently dropped these writes.

Actually, on further investigation, we've always had a read_safe() test
for PV, which means that PV guests have always unilaterally seen #GP. 
Can we retain that behaviour please?

~Andrew

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