Re: [PATCH v2 10/10] KVM: x86: deprecate KVM_ASYNC_PF_SEND_ALWAYS

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 25/05/20 16:41, Vitaly Kuznetsov wrote:
>> Concerns were expressed around APF events delivery when CPU is not
>> in user mode (KVM_ASYNC_PF_SEND_ALWAYS), e.g.
>> https://lore.kernel.org/kvm/ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org/
>> 
>> 'Page ready' events are already free from '#PF abuse' but 'page not ready'
>> notifications still go through #PF (to be changed in the future). Make the
>> probability of running into issues when APF collides with regular #PF lower
>> by deprecating KVM_ASYNC_PF_SEND_ALWAYS. The feature doesn't seem to be
>> important enough for any particular workload to notice the difference.
>
> This has been disabled already in guest code, but I don't see a
> particular reason to deprecate it in the host too.  Supporting it on the
> host is just one line of code; if it's a problem *for the guest*, you
> just don't use KVM_ASYNC_PF_SEND_ALWAYS.
>
> Also, note that #VE will always be delivered to the guest even at CPL0;
> the decision on whether to do sync or async page fault at CPL0 will move
> to the guest, but enabling #VE will probably _require_ the
> KVM_ASYNC_PF_SEND_ALWAYS bit to be set (and KVM_ASYNC_PF_DELIVERY_AS_INT
> as well).

I actually missed the fact that guest side is already disabled (I can
see it now in the queue), feel free to ignore this patch then.

-- 
Vitaly