Re: [PATCH v6 6/9] target/i386: query mshv accel for supported cpuids

[Magnus Kulke <magnuskulke@linux.microsoft.com>]

On Thu, Jun 25, 2026 at 05:16:44PM +0200, Paolo Bonzini wrote:
> On 4/16/26 14:11, Magnus Kulke wrote:
> > We implement mshv_get_supported_cpuid() and invoke it in
> > x86_cpu_get_supported_feature_word() retrieve the cpu features that the
> > host is supporting. Initially we mask the virtualization capabilitities
> > potentially we might need to mask more in the future.
> 
> Hopefully not - the difference between what the host and the guest support
> is not something that QEMU can know.
> 
> Apart from that, there's nothing that stands out so I'm applying the series.
> 
> Paolo

I think that is something that we will probably have to refine more.
Intuitively I would also say that the VMM shouldn't be concerned much about
host capabilities, however -cpu host has some interesting implications for a
VMM that is driving msft hypervisor. I understand some features (CET I think?)
would have an impact the instruction emulator. For KVM that would be in the
kernel, but for MSHV (and HVF, WHPX I think) that's userland.

We're still discussing with the hypervisor and kernel team what the proper
approach is, how we can reconcile cpuids with the flags that we pass at
partition creation and how much awareness should exist in the VMM. I can
imagine that we'll take a conservative approach and end up enumerating
the features that we support in QEMU (and extend them in future releases
if required).

thx,

magnus