Re: KVM/arm64: SPE: Translate VA to IPA on a stage 2 fault instead of pinning VM memory

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: Will Deacon <will@kernel.org>,
	maz@kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: KVM/arm64: SPE: Translate VA to IPA on a stage 2 fault instead of pinning VM memory
Date: Wed, 27 Jul 2022 12:00:10 +0100	[thread overview]
Message-ID: <YuEauq4sevyQAYpF@monolith.localdoman> (raw)
In-Reply-To: <YuApmZFdZzTi5ROu@google.com>

Hi Oliver,

On Tue, Jul 26, 2022 at 10:51:21AM -0700, Oliver Upton wrote:
> Hi Alex,
> 
> On Mon, Jul 25, 2022 at 11:06:24AM +0100, Alexandru Elisei wrote:
> 
> [...]
> 
> I'm also a bit confused on how we would manage to un-pin memory on the
> way out with this. The guest is free to muck with the stage 1 and could
> cause the SPU to spew a bunch of stage 2 aborts if it wanted to be
> annoying. One way to tackle it would be to only allow a single
> root-to-target walk to be pinned by a vCPU at a time. Any time a new
> stage 2 abort comes from the SPU, we un-pin the old walk and pin the new
> one instead.

On the topic of memory unpinning, for a well behaved guest I belive that should
be done the next time the buffer is pinned. The buffer can (and should!) be
drained when both the buffer and sampling is disabled; unpinning the buffer when
profiling becomes disabled would lead to unnecessary stage 2 faults when
draining it.

That approach also means that KVM wouldn't have to do anything special for SPE
stage 2 faults.

Thanks,
Alex

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