Re: [PATCH v2 0/2] KVM: x86: expose direct stores instructions into VM.

[Paolo Bonzini]

On 30/11/18 21:41, Eric Northup wrote:
> On Mon, Nov 5, 2018 at 10:01 PM Liu Jingqi <jingqi.liu@xxxxxxxxx> wrote:
>>
>> Direct stores instructions MOVDIRI and MOVDIR64B will be available in
>> Tremont and other future x86 processors,
>> and need to be exposed to guest VM.
> 
> It seems like KVM's emulator should be able to complete these
> instructions to emulated MMIO before exposing CPUID to guests in any
> default or supported configurations.
> 
> It'll be much simpler for usermode to implement that property if the
> KVM-reported supported CPUID table doesn't get updated before the KVM
> emulator does.

We already do not support emulation for many CPUID bits we expose (AVX
and AVX512 above all), so that's not a huge problem.

I suppose these instructions are unlikely to be used on MMIO areas such
as legacy VGA VRAM.

Paolo