linux-kernel - Re: [PATCH RFCv2 0/9] kvm/arm64: Support Async Page Fault

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Message-ID: <eb2a2291-85b0-a39a-96d1-58e4652771ba@redhat.com>
Date:   Wed, 27 May 2020 18:10:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>, Gavin Shan <gshan@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, catalin.marinas@....com, james.morse@....com,
        suzuki.poulose@....com, drjones@...hat.com, eric.auger@...hat.com,
        aarcange@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH RFCv2 0/9] kvm/arm64: Support Async Page Fault

On 27/05/20 09:48, Marc Zyngier wrote:
> 
> My own question is whether this even makes any sense 10 years later.
> The HW has massively changed, and this adds a whole lot of complexity
> to both the hypervisor and the guest.

It still makes sense, but indeed it's for different reasons.  One
example is host page cache sharing, where (parts of) the host page cache
are visible to the guest.  In this context, async page faults are used
for any kind of host page faults, not just paging out memory due to
overcommit.

But I agree that it is very very important to design the exception model
first, as we're witnessing in x86 land the problems with a poor design.
 Nothing major, but just pain all around.

Paolo

> It also plays very ugly games
> with the exception model, which doesn't give me the warm fuzzy feeling
> that it's going to be great.