linux-kernel - Re: [PATCH v7 04/11] KVM: pfncache: base offset check on khva rather than gpa

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Message-ID: <ZUV91k9cUZt9jTbm@google.com>
Date:   Fri, 3 Nov 2023 16:10:14 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     paul@....org
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paul Durrant <pdurrant@...zon.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 04/11] KVM: pfncache: base offset check on khva rather
 than gpa

On Thu, Nov 02, 2023, Paul Durrant wrote:
> On 31/10/2023 23:40, Sean Christopherson wrote:
> > On Mon, Oct 02, 2023, Paul Durrant wrote:
> > > From: Paul Durrant <pdurrant@...zon.com>
> > > 
> > > After a subsequent patch, the gpa may not always be set whereas khva will
> > > (as long as the cache valid flag is also set).
> > 
> > This holds true only because there are no users of KVM_GUEST_USES_PFN, and
> > because hva_to_pfn_retry() rather oddly adds the offset to a NULL khva.
> > 
> > I think it's time to admit using this to map PFNs into the guest is a bad idea
> > and rip out KVM_GUEST_USES_PFN before fully relying on khva.
> > 
> > https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com
> 
> Is this something you want me to fix?

Yes?  I don't want to snowball your series, but I also really don't like the
confusion that is introduced by relying on khva while KVM_GUEST_USES_PFN is still
a thing.

Can you give it a shot, and then holler if it's a bigger mess than I'm anticipating?
I'm assuming/hoping it's a relatively small, one-off patch, but I haven't actually
dug through in-depth to figure out what all needs to change.