linux-kernel - Re: [PATCH v2] KVM: x86: clean up conditions for asynchronous page fault handling

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Date:   Mon, 17 Jun 2019 21:23:18 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: clean up conditions for asynchronous page
 fault handling

2019-06-13 19:22+0200, Paolo Bonzini:
> Even when asynchronous page fault is disabled, KVM does not want to pause
> the host if a guest triggers a page fault; instead it will put it into
> an artificial HLT state that allows running other host processes while
> allowing interrupt delivery into the guest.
> 
> However, the way this feature is triggered is a bit confusing.
> First, it is not used for page faults while a nested guest is
> running: but this is not an issue since the artificial halt
> is completely invisible to the guest, either L1 or L2.  Second,
> it is used even if kvm_halt_in_guest() returns true; in this case,
> the guest probably should not pay the additional latency cost of the
> artificial halt, and thus we should handle the page fault in a
> completely synchronous way.
> 
> By introducing a new function kvm_can_deliver_async_pf, this patch
> commonizes the code that chooses whether to deliver an async page fault
> (kvm_arch_async_page_not_present) and the code that chooses whether a
> page fault should be handled synchronously (kvm_can_do_async_pf).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>