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

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Date:   Fri, 18 Nov 2022 17:12:49 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Will Deacon <will@...nel.org>
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>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for
 exclusive table walks

On Fri, Nov 18, 2022 at 12:19:50PM +0000, Will Deacon wrote:
> On Thu, Nov 17, 2022 at 06:23:23PM +0000, Oliver Upton wrote:
> > 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())?
> 
> Yes, that's what I had in mind. WARN is fatal at EL2, so I think it's
> better to fail the pgtable operation rather than bring down the entire
> machine by default.

Duh, I forgot WARNs really do go boom at EL2. Yeah, in that case it'd be
best to let the caller clean up the mess.

> > If having this is a strong motivator I can do a v4.
> 
> It's a really minor point, so I'll leave it up to you guys.

Sold (sorry I wasn't following before). v4 on the way.

--
Thanks,
Oliver