Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for exclusive table walks

From: Oliver Upton <oliver.upton@linux.dev>
To: Will Deacon <will@kernel.org>
Cc: kvm@vger.kernel.org, Marc Zyngier <maz@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,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for exclusive table walks
Date: Thu, 17 Nov 2022 18:23:23 +0000	[thread overview]
Message-ID: <Y3Z8G3aCuRzzoq5e@google.com> (raw)
In-Reply-To: <20221117174951.GA2916@willie-the-truck>

Hi Will,

Thanks for having a look.

On Thu, Nov 17, 2022 at 05:49:52PM +0000, Will Deacon wrote:
> On Wed, Nov 16, 2022 at 04:56:55PM +0000, Oliver Upton wrote:

[...]

> > -static inline void kvm_pgtable_walk_begin(void) {}
> > -static inline void kvm_pgtable_walk_end(void) {}
> > +static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
> > +{
> > +	/*
> > +	 * Due to the lack of RCU (or a similar protection scheme), only
> > +	 * non-shared table walkers are allowed in the hypervisor.
> > +	 */
> > +	WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED);
> > +}
> 
> I think it would be better to propagate the error to the caller rather
> than WARN here.

I'd really like to warn somewhere though since we're rather fscked at
this point. Keeping that WARN close to the exceptional condition would
help w/ debugging.

Were you envisioning bubbling the error all the way back up (i.e. early
return from kvm_pgtable_walk())?

I had really only intended these to indirect lock acquisition/release,
so the error handling on the caller side feels weird:

  static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
  {
  	if (WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED))
		return -EPERM;

	return 0;
  }

  r = kvm_pgtable_walk_begin()
  if (r)
  	return r;

  r = _kvm_pgtable_walk();
  kvm_pgtable_walk_end();

> Since you're rejigging things anyway, can you have this
> function return int?

If having this is a strong motivator I can do a v4.

--
Thanks,
Oliver
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