Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core | Date | Wed, 13 Oct 2021 16:14:23 +0200 |
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On Wed, Oct 13 2021 at 14:26, Paolo Bonzini wrote:
> On 13/10/21 12:14, Andy Lutomirski wrote: >>> I think it's simpler to always wait for #NM, it will only happen >>> once per vCPU. In other words, even if the guest clears XFD before >>> it generates #NM, the guest_fpu's XFD remains nonzero and an #NM >>> vmexit is possible. After #NM the guest_fpu's XFD is zero; then >>> passthrough can happen and the #NM vmexit trap can be disabled. >> >> This will stop being at all optimal when Intel inevitably adds >> another feature that uses XFD. In the potentially infinite window in >> which the guest manages XFD and #NM on behalf of its userspace and >> when the guest allocates the other hypothetical feature, all the #NMs >> will have to be trapped by KVM. > > The reason is that it's quite common to simply let the guest see all > CPUID bits that KVM knows about.
On fleets the cpu features exposed to guests matter a lot to ensure migratability and I would be surprised when such a feature would just be universally available to anyone.
Thanks,
tglx
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