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From: Oliver Upton To: David Matlack Subject: Re: [PATCH 08/14] KVM: arm64: Protect page table traversal with RCU Message-ID: References: <20220830194132.962932-1-oliver.upton@linux.dev> <20220830194132.962932-9-oliver.upton@linux.dev> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Cc: kvm@vger.kernel.org, Marc Zyngier , linux-kernel@vger.kernel.org, Catalin Marinas , Ben Gardon , Paolo Bonzini , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Wed, Sep 07, 2022 at 02:47:08PM -0700, David Matlack wrote: > On Tue, Aug 30, 2022 at 07:41:26PM +0000, Oliver Upton wrote: > > The use of RCU is necessary to change the paging structures in parallel. > > Acquire and release an RCU read lock when traversing the page tables. > > > > Signed-off-by: Oliver Upton > > --- > > arch/arm64/include/asm/kvm_pgtable.h | 19 ++++++++++++++++++- > > arch/arm64/kvm/hyp/pgtable.c | 7 ++++++- > > 2 files changed, 24 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > > index 78fbb7be1af6..7d2de0a98ccb 100644 > > --- a/arch/arm64/include/asm/kvm_pgtable.h > > +++ b/arch/arm64/include/asm/kvm_pgtable.h > > @@ -578,9 +578,26 @@ enum kvm_pgtable_prot kvm_pgtable_stage2_pte_prot(kvm_pte_t pte); > > */ > > enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte); > > > > +#if defined(__KVM_NVHE_HYPERVISOR___) > > + > > Future readers will wonder why NVHE stubs out RCU support and how that > is even correct. Some comments here would be useful explain it. Good point. > > +static inline void kvm_pgtable_walk_begin(void) {} > > +static inline void kvm_pgtable_walk_end(void) {} > > + > > +#define kvm_dereference_ptep rcu_dereference_raw > > How does NVHE have access rcu_dereference_raw()? rcu_dereference_raw() is inlined and simply recasts the pointer into the kernel address space. Perhaps it is less confusing to template this on kvm_pte_read() to avoid polluting nVHE with an otherwise benign reference to RCU. > > + > > +#else /* !defined(__KVM_NVHE_HYPERVISOR__) */ > > + > > +#define kvm_pgtable_walk_begin rcu_read_lock > > +#define kvm_pgtable_walk_end rcu_read_unlock > > +#define kvm_dereference_ptep rcu_dereference > > + > > +#endif /* defined(__KVM_NVHE_HYPERVISOR__) */ > > + > > static inline kvm_pte_t kvm_pte_read(kvm_pte_t *ptep) > > { > > - return READ_ONCE(*ptep); > > + kvm_pte_t __rcu *p = (kvm_pte_t __rcu *)ptep; > > + > > + return READ_ONCE(*kvm_dereference_ptep(p)); > > What about all the other places where page table memory is accessed? > > If RCU is going to be used to protect page table memory, then all > accesses have to go under an RCU critical section. This means that page > table memory should only be accessed through __rcu annotated pointers > and dereferenced with rcu_dereference(). Let me play around with this a bit, as the annoying part is trying to sprinkle in RCU annotations w/o messing with nVHE. -- Thanks, Oliver _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm