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

Wang, Wei W
2021-12-14T06:06:58Z

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.

- 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