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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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: 

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. 

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.

-- 
Vitaly