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

Paolo Bonzini
2021-12-14T06:19:05Z

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
>