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Date: Thu, 17 Nov 2022 18:23:23 +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 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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