Re: [PATCH RFC] kvm: emulate avx vmovdq

[Tao Su <tao1.su@linux.intel.com>]

On Thu, Aug 22, 2024 at 08:39:19AM -0600, Keith Busch wrote:
> On Thu, Aug 22, 2024 at 03:22:35PM +0800, Tao Su wrote:
> > On Tue, Aug 20, 2024 at 04:04:31PM -0700, Keith Busch wrote:
> > > +	if (map == 1 && !v)
> > > +		return avx_0f_table[ctxt->b];
> > > +	return (struct opcode){.flags = NotImpl};
> > 
> > Can we check whether the host supports AVX? I.e. if the host does not support
> > AVX, set NotImpl. I am thinking that if the host does not support AVX, perhaps
> > the guest executing AVX instructions will cause the host to panic, because the
> > host will execute AVX instructions during the simulation.
> > 
> > Yeah if the host does not support AVX, it may not report AVX to the guest, but
> > the guest can always ignore the AVX check, such as the code in the commit.
> 
> That's a good thought. Here is how I rationalized not adding additional
> checks for it:
> 
> If the guest cpu doesn't support AVX, I think it should fail then and
> there rather than trap to the hypervisor running on the host, so this
> new code wouldn't get a chance to attempt emulating it. 
> 

Per SDM:
If YMM state management is not enabled by an operating systems, Intel AVX
instructions will #UD regardless of CPUID.1:ECX.AVX[bit 28].

Host and guest can set different xstates, so it has possibility to trigger,
i.e. host clears but guest sets XCR0[2]. But I don’t see this case can occur
now, so just ignore my concern if no one else wants to do that :-)

> In the case where the host doesn't support AVX, but the guest does
> support it, then I assume the VM is running on an emulated CPU and not
> using kvm acceleration anymore.
> 
> Anyway, I haven't tried it, so not entirely confident that's how this
> all works. I was mainly following the existing SSE emulations, which
> don't have CPU support checks either. I don't think it's a problem to
> add such checks though, so happy to do it if needed.