Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't set

From: Marc Zyngier <maz@kernel.org>
To: Ricardo Koller <ricarkol@google.com>
Cc: kvm@vger.kernel.org, Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't set
Date: Wed, 30 Nov 2022 08:21:17 +0000	[thread overview]
Message-ID: <87pmd4ua2q.wl-maz@kernel.org> (raw)
In-Reply-To: <Y4awiKLuKORZmU2z@google.com>

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).

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.
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