Re: [PATCH 1/2] KVM: x86: async_pf: remove support for KVM_ASYNC_PF_SEND_ALWAYS

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 17, 2025, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Wed, Nov 27, 2024, Nikita Kalyazin wrote:
> >> 3a7c8fafd1b42adea229fd204132f6a2fb3cd2d9 ("x86/kvm: Restrict
> >> ASYNC_PF to user space") stopped setting KVM_ASYNC_PF_SEND_ALWAYS in
> >> Linux guests.  While the flag can still be used by legacy guests, the
> >> mechanism is best effort so KVM is not obliged to use it.
> >
> > What's the actual motivation to remove it from KVM?  I agreed KVM isn't required
> > to honor KVM_ASYNC_PF_SEND_ALWAYS from a guest/host ABI perspective, but that
> > doesn't mean that dropping a feature has no impact.  E.g. it's entirely possible
> > removing this support could negatively affect a workload running on an old kernel.
> >
> > Looking back at the discussion[*] where Vitaly made this suggestion, I don't see
> > anything that justifies dropping this code.  It costs KVM practically nothing to
> > maintain this code.
> >
> > [*] https://lore.kernel.org/all/20241118130403.23184-1-kalyazin@amazon.com
> >
> 
> How old is old? :-)
> 
> Linux stopped using KVM_ASYNC_PF_SEND_ALWAYS in v5.8: 

5.8 is practically a baby.  Maybe a toddler :-)

> commit 3a7c8fafd1b42adea229fd204132f6a2fb3cd2d9
> Author: Thomas Gleixner <tglx@linutronix.de>
> Date:   Fri Apr 24 09:57:56 2020 +0200
> 
>     x86/kvm: Restrict ASYNC_PF to user space
> 
> and I was under the impression other OSes never used KVM asynchronous
> page-fault in the first place (not sure about *BSDs though but certainly
> not Windows). As Nikita's motivation for the patch was "to avoid the
> overhead ... in case of kernel-originated faults" I suggested we start
> by simplifyign the code to not care about 'send_user_only' at all. 

In practice, I don't think it's a meaningful simplification.  There are other
scenarios where KVM shouldn't inject an async #PF, so kvm_can_deliver_async_pf()
itself isn't going anywhere.

AFAICT, what Nikita actually wants is a way to disable host-side async #PF, e.g.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f97d4d435e7f..d461e1b5489c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13411,7 +13411,8 @@ bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)
                     kvm_is_exception_pending(vcpu)))
                return false;
 
-       if (kvm_hlt_in_guest(vcpu->kvm) && !kvm_can_deliver_async_pf(vcpu))
+       if ((kvm_hlt_in_guest(vcpu->kvm) || kvm_only_pv_async_pf(vcpu->kvm)) &&
+           !kvm_can_deliver_async_pf(vcpu))
                return false;
 
        /*

> We can keep the code around, I guess, but with no plans to re-introduce
> KVM_ASYNC_PF_SEND_ALWAYS usage to Linux I still believe it would be good
> to set a deprecation date.