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[34.82.181.220]) by smtp.gmail.com with ESMTPSA id y186-20020a6232c3000000b00535e46171c1sm8503863pfy.117.2022.09.13.17.49.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Sep 2022 17:49:30 -0700 (PDT) Date: Tue, 13 Sep 2022 17:49:27 -0700 From: Ricardo Koller To: Oliver Upton Cc: Marc Zyngier , James Morse , Alexandru Elisei , Suzuki K Poulose , Catalin Marinas , Will Deacon , Quentin Perret , Reiji Watanabe , David Matlack , Ben Gardon , Paolo Bonzini , Gavin Shan , Peter Xu , Sean Christopherson , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 09/14] KVM: arm64: Free removed stage-2 tables in RCU callback Message-ID: References: <20220830194132.962932-1-oliver.upton@linux.dev> <20220830194132.962932-10-oliver.upton@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220830194132.962932-10-oliver.upton@linux.dev> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Oliver, On Tue, Aug 30, 2022 at 07:41:27PM +0000, Oliver Upton wrote: > There is no real urgency to free a stage-2 subtree that was pruned. > Nonetheless, KVM does the tear down in the stage-2 fault path while > holding the MMU lock. > > Free removed stage-2 subtrees after an RCU grace period. To guarantee > all stage-2 table pages are freed before killing a VM, add an > rcu_barrier() to the flush path. > > Signed-off-by: Oliver Upton > --- > arch/arm64/kvm/mmu.c | 35 ++++++++++++++++++++++++++++++++++- > 1 file changed, 34 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > index 91521f4aab97..265951c05879 100644 > --- a/arch/arm64/kvm/mmu.c > +++ b/arch/arm64/kvm/mmu.c > @@ -97,6 +97,38 @@ static void *stage2_memcache_zalloc_page(void *arg) > return kvm_mmu_memory_cache_alloc(mc); > } > > +#define STAGE2_PAGE_PRIVATE_LEVEL_MASK GENMASK_ULL(2, 0) > + > +static inline unsigned long stage2_page_private(u32 level, void *arg) > +{ > + unsigned long pvt = (unsigned long)arg; > + > + BUILD_BUG_ON(KVM_PGTABLE_MAX_LEVELS > STAGE2_PAGE_PRIVATE_LEVEL_MASK); > + WARN_ON_ONCE(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK); If the pgt pointer (arg) is not aligned for some reason, I think it might be better to BUG_ON(). Alternatively, why not trying to pass a new struct (with level and arg) that's freed by the rcu callback. > + > + return pvt | level; > +} > + > +static void stage2_free_removed_table_rcu_cb(struct rcu_head *head) > +{ > + struct page *page = container_of(head, struct page, rcu_head); > + unsigned long pvt = page_private(page); > + void *arg = (void *)(pvt & ~STAGE2_PAGE_PRIVATE_LEVEL_MASK); > + u32 level = (u32)(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK); > + void *pgtable = page_to_virt(page); > + > + kvm_pgtable_stage2_free_removed(pgtable, level, arg); > +} > + > +static void stage2_free_removed_table(void *pgtable, u32 level, void *arg) > +{ > + unsigned long pvt = stage2_page_private(level, arg); > + struct page *page = virt_to_page(pgtable); > + > + set_page_private(page, (unsigned long)pvt); > + call_rcu(&page->rcu_head, stage2_free_removed_table_rcu_cb); > +} > + > static void *kvm_host_zalloc_pages_exact(size_t size) > { > return alloc_pages_exact(size, GFP_KERNEL_ACCOUNT | __GFP_ZERO); > @@ -627,7 +659,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = { > .zalloc_page = stage2_memcache_zalloc_page, > .zalloc_pages_exact = kvm_host_zalloc_pages_exact, > .free_pages_exact = free_pages_exact, > - .free_removed_table = kvm_pgtable_stage2_free_removed, > + .free_removed_table = stage2_free_removed_table, > .get_page = kvm_host_get_page, > .put_page = kvm_host_put_page, > .page_count = kvm_host_page_count, > @@ -770,6 +802,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu) > if (pgt) { > kvm_pgtable_stage2_destroy(pgt); > kfree(pgt); > + rcu_barrier(); > } > } > > -- > 2.37.2.672.g94769d06f0-goog >