Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl

[Paolo Bonzini]

On 12/14/21 07:06, Wang, Wei W wrote:
> On Monday, December 13, 2021 5:24 PM, Paolo Bonzini wrote:
> > There is no need for struct kvm_xsave2, because there is no need for a "size"
> > argument.
> >
> > - KVM_GET_XSAVE2 *is* needed, and it can expect a buffer as big as the return
> > value of KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2)
>
> Why would KVM_GET_XSAVE2 still be needed in this case?
>
> I'm thinking it would also be possible to reuse KVM_GET_XSAVE:
>
> - If userspace calls to KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2),
>   then KVM knows that the userspace is a new version and it works with larger xsave buffer using the "size" that it returns via KVM_CAP_XSAVE2.
>   So we can add a flag "kvm->xsave2_enabled", which gets set upon userspace checks KVM_CAP_XSAVE2.

You can use KVM_ENABLE_CAP(KVM_CAP_XSAVE2) for that, yes.  In that case 
you don't need KVM_GET_XSAVE2.

Paolo

> - On KVM_GET_XSAVE, if "kvm->xsave2_enabled" is set,
>   then KVM allocates buffer to load xstates and copies the loaded xstates data to the userspace buffer
>   using the "size" that was returned to userspace on KVM_CAP_XSAVE2.
>   If "kvm->xsave2_enabled" isn't set, using the legacy "4KB" size.
>
> Thanks,
> Wei