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

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Message-ID: <Y4awiKLuKORZmU2z@google.com>
Date:   Tue, 29 Nov 2022 17:23:20 -0800
From:   Ricardo Koller <ricarkol@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: arm64: Don't serialize if the access flag isn't
 set

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.

> 
> --
> Thanks,
> Oliver