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[34.82.181.220]) by smtp.gmail.com with ESMTPSA id w15-20020a17090a15cf00b00202618f0df4sm4603848pjd.0.2022.10.28.11.43.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Oct 2022 11:43:36 -0700 (PDT) Date: Fri, 28 Oct 2022 11:43:33 -0700 From: Ricardo Koller To: Oliver Upton Cc: Marc Zyngier , James Morse , Alexandru Elisei , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, Reiji Watanabe , David Matlack , Quentin Perret , Ben Gardon , Gavin Shan , Peter Xu , Will Deacon , Sean Christopherson , kvmarm@lists.linux.dev Subject: Re: [PATCH v2 06/15] KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make Message-ID: References: <20221007232818.459650-1-oliver.upton@linux.dev> <20221007232818.459650-7-oliver.upton@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Oct 28, 2022 at 11:41:57AM -0700, Ricardo Koller wrote: > On Fri, Oct 07, 2022 at 11:28:09PM +0000, Oliver Upton wrote: > > The break-before-make sequence is a bit annoying as it opens a window > > wherein memory is unmapped from the guest. KVM should replace the PTE > > as quickly as possible and avoid unnecessary work in between. > > > > Presently, the stage-2 map walker tears down a removed table before > > installing a block mapping when coalescing a table into a block. As the > > removed table is no longer visible to hardware walkers after the > > DSB+TLBI, it is possible to move the remaining cleanup to happen after > > installing the new PTE. > > > > Reshuffle the stage-2 map walker to install the new block entry in > > the pre-order callback. Unwire all of the teardown logic and replace > > it with a call to kvm_pgtable_stage2_free_removed() after fixing > > the PTE. The post-order visitor is now completely unnecessary, so drop > > it. Finally, touch up the comments to better represent the now > > simplified map walker. > > > > Note that the call to tear down the unlinked stage-2 is indirected > > as a subsequent change will use an RCU callback to trigger tear down. > > RCU is not available to pKVM, so there is a need to use different > > implementations on pKVM and non-pKVM VMs. > > > > Signed-off-by: Oliver Upton > > --- > > arch/arm64/include/asm/kvm_pgtable.h | 3 + > > arch/arm64/kvm/hyp/nvhe/mem_protect.c | 6 ++ > > arch/arm64/kvm/hyp/pgtable.c | 84 ++++++++------------------- > > arch/arm64/kvm/mmu.c | 8 +++ > > 4 files changed, 40 insertions(+), 61 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > > index 71b7d154b78a..c33edcf36b5b 100644 > > --- a/arch/arm64/include/asm/kvm_pgtable.h > > +++ b/arch/arm64/include/asm/kvm_pgtable.h > > @@ -77,6 +77,8 @@ static inline bool kvm_level_supports_block_mapping(u32 level) > > * allocation is physically contiguous. > > * @free_pages_exact: Free an exact number of memory pages previously > > * allocated by zalloc_pages_exact. > > + * @free_removed_table: Free a removed paging structure by unlinking and > > + * dropping references. > > * @get_page: Increment the refcount on a page. > > * @put_page: Decrement the refcount on a page. When the > > * refcount reaches 0 the page is automatically > > @@ -95,6 +97,7 @@ struct kvm_pgtable_mm_ops { > > void* (*zalloc_page)(void *arg); > > void* (*zalloc_pages_exact)(size_t size); > > void (*free_pages_exact)(void *addr, size_t size); > > + void (*free_removed_table)(void *addr, u32 level); > > void (*get_page)(void *addr); > > void (*put_page)(void *addr); > > int (*page_count)(void *addr); > > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > index d21d1b08a055..735769886b55 100644 > > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c > > @@ -79,6 +79,11 @@ static void host_s2_put_page(void *addr) > > hyp_put_page(&host_s2_pool, addr); > > } > > > > +static void host_s2_free_removed_table(void *addr, u32 level) > > +{ > > + kvm_pgtable_stage2_free_removed(&host_kvm.mm_ops, addr, level); > > +} > > + > > static int prepare_s2_pool(void *pgt_pool_base) > > { > > unsigned long nr_pages, pfn; > > @@ -93,6 +98,7 @@ static int prepare_s2_pool(void *pgt_pool_base) > > host_kvm.mm_ops = (struct kvm_pgtable_mm_ops) { > > .zalloc_pages_exact = host_s2_zalloc_pages_exact, > > .zalloc_page = host_s2_zalloc_page, > > + .free_removed_table = host_s2_free_removed_table, > > .phys_to_virt = hyp_phys_to_virt, > > .virt_to_phys = hyp_virt_to_phys, > > .page_count = hyp_page_count, > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index 363a5cce7e1a..02c33fccb178 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -746,16 +746,19 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > return 0; > > } > > > > +static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > + struct stage2_map_data *data); > > + > > static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx, > > struct stage2_map_data *data) > > { > > - if (data->anchor) > > - return 0; > > + struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops; > > + kvm_pte_t *childp = kvm_pte_follow(ctx->old, mm_ops); > > + int ret; > > > > if (!stage2_leaf_mapping_allowed(ctx, data)) > > return 0; > > > > - data->childp = kvm_pte_follow(ctx->old, ctx->mm_ops); > > kvm_clear_pte(ctx->ptep); > > > > /* > > @@ -764,8 +767,13 @@ static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx, > > * individually. > > */ > > kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu); > > - data->anchor = ctx->ptep; > > - return 0; > > + > > + ret = stage2_map_walk_leaf(ctx, data); > > + > > + mm_ops->put_page(ctx->ptep); > > + mm_ops->free_removed_table(childp, ctx->level + 1); > > This could save some cycles: > > if (stage2_pte_is_counted(childp) // typo: should be childp > mm_ops->free_removed_table(childp, ctx->level + 1); > > as coming back using the stage2_free_walker() requires some preparation, > and it does the exact same check anyawy. > > > + > > + return ret; > > } > > > > static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > @@ -775,13 +783,6 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > kvm_pte_t *childp; > > int ret; > > > > - if (data->anchor) { > > - if (stage2_pte_is_counted(ctx->old)) > > - mm_ops->put_page(ctx->ptep); > > - > > - return 0; > > - } > > - > > ret = stage2_map_walker_try_leaf(ctx, data); > > if (ret != -E2BIG) > > return ret; > > @@ -810,49 +811,14 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > return 0; > > } > > > > -static int stage2_map_walk_table_post(const struct kvm_pgtable_visit_ctx *ctx, > > - struct stage2_map_data *data) > > -{ > > - struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops; > > - kvm_pte_t *childp; > > - int ret = 0; > > - > > - if (!data->anchor) > > - return 0; > > - > > - if (data->anchor == ctx->ptep) { > > - childp = data->childp; > > - data->anchor = NULL; > > - data->childp = NULL; > > - ret = stage2_map_walk_leaf(ctx, data); > > - } else { > > - childp = kvm_pte_follow(ctx->old, mm_ops); > > - } > > - > > - mm_ops->put_page(childp); > > - mm_ops->put_page(ctx->ptep); > > - > > - return ret; > > -} > > - > > /* > > - * This is a little fiddly, as we use all three of the walk flags. The idea > > - * is that the TABLE_PRE callback runs for table entries on the way down, > > - * looking for table entries which we could conceivably replace with a > > - * block entry for this mapping. If it finds one, then it sets the 'anchor' > > - * field in 'struct stage2_map_data' to point at the table entry, before > > - * clearing the entry to zero and descending into the now detached table. > > - * > > - * The behaviour of the LEAF callback then depends on whether or not the > > - * anchor has been set. If not, then we're not using a block mapping higher > > - * up the table and we perform the mapping at the existing leaves instead. > > - * If, on the other hand, the anchor _is_ set, then we drop references to > > - * all valid leaves so that the pages beneath the anchor can be freed. > > + * The TABLE_PRE callback runs for table entries on the way down, looking > > + * for table entries which we could conceivably replace with a block entry > > + * for this mapping. If it finds one it replaces the entry and calls > > + * kvm_pgtable_mm_ops::free_removed_table() to tear down the detached table. > > * > > - * Finally, the TABLE_POST callback does nothing if the anchor has not > > - * been set, but otherwise frees the page-table pages while walking back up > > - * the page-table, installing the block entry when it revisits the anchor > > - * pointer and clearing the anchor to NULL. > > + * Otherwise, the LEAF callback performs the mapping at the existing leaves > > + * instead. > > */ > > static int stage2_map_walker(const struct kvm_pgtable_visit_ctx *ctx, > > enum kvm_pgtable_walk_flags visit) > > @@ -864,11 +830,9 @@ static int stage2_map_walker(const struct kvm_pgtable_visit_ctx *ctx, > > return stage2_map_walk_table_pre(ctx, data); > > case KVM_PGTABLE_WALK_LEAF: > > return stage2_map_walk_leaf(ctx, data); > > - case KVM_PGTABLE_WALK_TABLE_POST: > > - return stage2_map_walk_table_post(ctx, data); > > + default: > > + return -EINVAL; > > } > > - > > - return -EINVAL; > > } > > > > int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size, > > @@ -885,8 +849,7 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size, > > struct kvm_pgtable_walker walker = { > > .cb = stage2_map_walker, > > .flags = KVM_PGTABLE_WALK_TABLE_PRE | > > - KVM_PGTABLE_WALK_LEAF | > > - KVM_PGTABLE_WALK_TABLE_POST, > > + KVM_PGTABLE_WALK_LEAF, > > .arg = &map_data, > > }; > > > > @@ -916,8 +879,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size, > > struct kvm_pgtable_walker walker = { > > .cb = stage2_map_walker, > > .flags = KVM_PGTABLE_WALK_TABLE_PRE | > > - KVM_PGTABLE_WALK_LEAF | > > - KVM_PGTABLE_WALK_TABLE_POST, > > + KVM_PGTABLE_WALK_LEAF, > > .arg = &map_data, > > }; > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > index c9a13e487187..04a25319abb0 100644 > > --- a/arch/arm64/kvm/mmu.c > > +++ b/arch/arm64/kvm/mmu.c > > @@ -102,6 +102,13 @@ static void *kvm_host_zalloc_pages_exact(size_t size) > > return alloc_pages_exact(size, GFP_KERNEL_ACCOUNT | __GFP_ZERO); > > } > > > > +static struct kvm_pgtable_mm_ops kvm_s2_mm_ops; > > + > > +static void stage2_free_removed_table(void *addr, u32 level) > > +{ > > + kvm_pgtable_stage2_free_removed(&kvm_s2_mm_ops, addr, level); > > +} > > + > > static void kvm_host_get_page(void *addr) > > { > > get_page(virt_to_page(addr)); > > @@ -627,6 +634,7 @@ 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 = stage2_free_removed_table, > > .get_page = kvm_host_get_page, > > .put_page = kvm_host_put_page, > > .page_count = kvm_host_page_count, > > -- > > 2.38.0.rc1.362.ged0d419d3c-goog > >