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

[Sean Christopherson]

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@xxxxxxxxxx