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Date:   Wed, 30 Nov 2022 15:21:16 -0800
From:   Ricardo Koller <ricarkol@google.com>
To:     Marc Zyngier <maz@kernel.org>
Cc:     Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        kvm@vger.kernel.org, kvmarm@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't
 set
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On Wed, Nov 30, 2022 at 08:21:17AM +0000, Marc Zyngier wrote:
> On Wed, 30 Nov 2022 01:23:20 +0000,
> Ricardo Koller <ricarkol@google.com> wrote:
> > 
> > On Tue, Nov 29, 2022 at 09:15:21PM +0000, Oliver Upton wrote:
> > > Hi Ricardo,
> > > 
> > > Thanks for having a look.
> > > 
> > > On Tue, Nov 29, 2022 at 12:52:12PM -0800, Ricardo Koller wrote:
> > > > On Tue, Nov 29, 2022 at 07:19:44PM +0000, Oliver Upton wrote:
> > > 
> > > [...]
> > > 
> > > > > +	ret = stage2_update_leaf_attrs(pgt, addr, 1, KVM_PTE_LEAF_ATTR_LO_S2_AF, 0,
> > > > > +				       &pte, NULL, 0);
> > > > > +	if (!ret)
> > > > > +		dsb(ishst);
> > > > 
> > > > At the moment, the only reason for stage2_update_leaf_attrs() to not
> > > > update the PTE is if it's not valid:
> > > > 
> > > > 	if (!kvm_pte_valid(pte))
> > > > 			return 0;
> > > > 
> > > > I guess you could check that as well:
> > > > 
> > > > +	if (!ret || kvm_pte_valid(pte))
> > > > +		dsb(ishst);
> > > 
> > > Thanks for catching this.
> > > 
> > > Instead of pivoting on the returned PTE value, how about we return
> > > -EAGAIN from the early return in stage2_attr_walker()? It would better
> > > match the pattern used elsewhere in the pgtable code.
> > 
> > That works, although I would use another return code (e.g., EINVAL)? as
> > that's not exactly a "try again" type of error.
> 
> EINVAL usually is an indication of something that went horribly wrong.
> 
> But is that really a failure mode? Here, failing to update the PTE
> should not be considered a failure, but just a benign race: access
> fault being taken on a CPU and the page being evicted on another (not
> unlikely, as the page was marked old before).

I see, I agree, what you describe not look like a failure.

> 
> And if I'm correct above, this is definitely a "try again" situation:
> you probably won't take the same type of fault the second time though.
> 
> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.
>