Re: [PATCH] KVM: arm64: Ensure I-cache isolation between vcpus of a same VM

From: Catalin Marinas <catalin.marinas@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	kernel-team@android.com, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>
Subject: Re: [PATCH] KVM: arm64: Ensure I-cache isolation between vcpus of a same VM
Date: Sat, 6 Mar 2021 14:15:48 +0000	[thread overview]
Message-ID: <20210306141546.GB2932@arm.com> (raw)
In-Reply-To: <877dmksgaw.wl-maz@kernel.org>

On Sat, Mar 06, 2021 at 10:54:47AM +0000, Marc Zyngier wrote:
> On Fri, 05 Mar 2021 19:07:09 +0000,
> Catalin Marinas <catalin.marinas@arm.com> wrote:
> > 
> > On Wed, Mar 03, 2021 at 04:45:05PM +0000, Marc Zyngier wrote:
> > > It recently became apparent that the ARMv8 architecture has interesting
> > > rules regarding attributes being used when fetching instructions
> > > if the MMU is off at Stage-1.
> > > 
> > > In this situation, the CPU is allowed to fetch from the PoC and
> > > allocate into the I-cache (unless the memory is mapped with
> > > the XN attribute at Stage-2).
> > 
> > Digging through the ARM ARM is hard. Do we have this behaviour with FWB
> > as well?
> 
> The ARM ARM doesn't seem to mention FWB at all when it comes to
> instruction fetch, which is sort of expected as it only covers the
> D-side. I *think* we could sidestep this when CTR_EL0.DIC is set
> though, as the I-side would then snoop the D-side.

Not sure this helps. CTR_EL0.DIC refers to the need for maintenance to
PoU while the SCTLR_EL1.M == 0 causes the I-cache to fetch from PoC. I
don't think I-cache snooping the D-cache would happen to the PoU when
the S1 MMU is off.

My reading of D4.4.4 is that when SCTLR_EL1.M == 0 both I and D accesses
are Normal Non-cacheable with a note in D4.4.6 that Non-cacheable
accesses may be held in the I-cache.

The FWB rules on combining S1 and S2 says that Normal Non-cacheable at
S1 is "upgraded" to cacheable. This should happen irrespective of
whether the S1 MMU is on or off and should apply to both I and D
accesses (since it does not explicitly says). So I think we could skip
this IC IALLU when FWB is present.

The same logic should apply when the VMM copies the VM text. With FWB,
we probably only need D-cache maintenance to PoU and only if
CTR_EL0.IDC==0. I haven't checked what the code currently does.

-- 
Catalin

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