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[34.140.183.197]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a1785398b5sm156955e9.2.2026.10.05.08.02.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Oct 2026 08:02:12 -0700 (PDT) Date: Mon, 5 Oct 2026 16:02:06 +0100 From: Vincent Donnefort To: Mostafa Saleh Cc: linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, maz@kernel.org, oupton@kernel.org, seiden@linux.ibm.com, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, tabba@google.com, sebastianene@google.com, keirf@google.com, qperret@google.com, linu.cherian@arm.com Subject: Re: [PATCH v3 2/2] KVM: arm64: Support BBM level 3 Message-ID: References: <20260904132855.638117-1-smostafa@google.com> <20260904132855.638117-3-smostafa@google.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260904132855.638117-3-smostafa@google.com> On Fri, Sep 04, 2026 at 01:28:55PM +0000, Mostafa Saleh wrote: > If the system supports hardware Break-Before-Make (BBM) level 3, use it > to replace stage-2 PTEs directly. Otherwise, fall back to the software > BBM sequence. > > For BBML3 the sequence is: > 1) Get a reference count on the containing table for the new PTE. > 2) Atomically update the PTE with the new valid descriptor. > 3) Invalidate the TLB for the old PTE. > 4) Drop the reference count holding the old PTE. > > Add 2 helpers: > 1) kvm_pgtable_use_bbml3(): Checks for the architecture requirement > for BBML3. > > 2) stage2_use_bbml3(): Extra checks added by SW design (FWB and DIC) > - As BBML3 will update the PTE atomically, it can only know it > raced with another core at the point of the cmpxchg failing, > unlike the SW implementation which locks the PTE first. > And as we must issue CMOs to the new mapped page before the > update, that means with BBML3 racing cores will issue redundant > CMOs. > > Signed-off-by: Mostafa Saleh > --- > arch/arm64/kvm/hyp/pgtable.c | 111 ++++++++++++++++++++++++++++------- > 1 file changed, 90 insertions(+), 21 deletions(-) > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index d670da8882a5..a9ba761e9a01 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -82,6 +82,27 @@ static bool kvm_pte_table(kvm_pte_t pte, s8 level) > return FIELD_GET(KVM_PTE_TYPE, pte) == KVM_PTE_TYPE_TABLE; > } > > +/* > + * Check if BBML3 can be used for this PTE update. > + * Fallback to software break-before-make for leaf-to-leaf changes. > + */ > +static bool kvm_pgtable_use_bbml3(const struct kvm_pgtable_visit_ctx *ctx, > + kvm_pte_t new) > +{ > + if (!system_supports_bbml3()) > + return false; > + > + if (!kvm_pte_valid(ctx->old) || !kvm_pte_valid(new)) > + return false; > + > + /* Block <-> Table is ok. */ > + if (kvm_pte_table(new, ctx->level) || > + kvm_pte_table(ctx->old, ctx->level)) > + return true; > + > + return false; > +} > + > static kvm_pte_t *kvm_pte_follow(kvm_pte_t pte, struct kvm_pgtable_mm_ops *mm_ops) > { > return mm_ops->phys_to_virt(kvm_pte_to_phys(pte)); > @@ -835,25 +856,46 @@ static void stage2_clean_old_pte(const struct kvm_pgtable_visit_ctx *ctx, > mm_ops->put_page(ctx->ptep); > } > > +/* > + * Don't use bbml3 for stage-2 if FWB or DIC are not supported > + * as that means racing cores will issue duplicate CMOs. > + */ > +static bool stage2_use_bbml3(const struct kvm_pgtable_visit_ctx *ctx, > + kvm_pte_t new) > +{ > + if (!cpus_have_final_cap(ARM64_HAS_STAGE2_FWB) || > + !cpus_have_final_cap(ARM64_HAS_CACHE_DIC)) > + return false; > + > + return kvm_pgtable_use_bbml3(ctx, new); > +} > + > /** > * stage2_try_break_pte() - Invalidates a pte according to the > * 'break-before-make' requirements of the > - * architecture. > + * architecture, if BBML3 is supported it > + * will be used and this function won't > + * break the PTE. > * > * @ctx: context of the visited pte. > * @mmu: stage-2 mmu > + * @new: New pte installed in make. > * > - * Returns: true if the pte was successfully broken. > + * Returns: true if the pte was successfully broken or BBML3 is used. > * > * If the removed pte was valid, performs the necessary serialization and TLB > * invalidation for the old value. For counted ptes, drops the reference count > * on the containing table page. > */ > static bool stage2_try_break_pte(const struct kvm_pgtable_visit_ctx *ctx, > - struct kvm_s2_mmu *mmu) > + struct kvm_s2_mmu *mmu, kvm_pte_t new) > { > kvm_pte_t locked_pte; > > + /* All handled in stage2_make_pte() */ > + if (stage2_use_bbml3(ctx, new)) > + return true; > + Wouldn't it be easier to keep try_break_pte/make_pte to the !bbml3 case and to just create a make_pte_bbml3() variant to be called when stage2_use_bbml3()? if (!stage2_use_bbml3()) { if (stage2_try_break_pte()) return -EAGAIN; stage2_make_pte(); } else { if (stage2_make_pte_bbml3()) return -EAGAIN; } I believe also, the error path would look less weird as we catch an error in make_pte() but without reverting the break_pte() (even if it is correct right now). And perhaps you could introduce a function that does both break/make (stage2_update_pte()?) called by both stage2_split_walker() and stage2_map_walk_leaf(). This would avoid repeating the error path. Otherwise, everything looks functional to me. -- Vincent > if (stage2_pte_is_locked(ctx->old)) { > /* > * Should never occur if this walker has exclusive access to the > @@ -873,16 +915,37 @@ static bool stage2_try_break_pte(const struct kvm_pgtable_visit_ctx *ctx, > return true; > } > > -static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t new) > +static bool stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu, > + kvm_pte_t new) > { > struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops; > > - WARN_ON(!stage2_pte_is_locked(*ctx->ptep)); > - > if (stage2_pte_is_counted(new)) > mm_ops->get_page(ctx->ptep); > > + if (stage2_use_bbml3(ctx, new)) { > + if (!kvm_pgtable_walk_shared(ctx)) { > + /* > + * stage2_try_set_pte() uses WRITE_ONCE for non-shared walks, > + * lacking release semantics used in the software BBM case. > + */ > + smp_wmb(); > + } > + > + if (!stage2_try_set_pte(ctx, new)) { > + /* Raced with another core. */ > + if (stage2_pte_is_counted(new)) > + mm_ops->put_page(ctx->ptep); > + return false; > + } > + > + stage2_clean_old_pte(ctx, mmu); > + return true; > + } > + > + WARN_ON(!stage2_pte_is_locked(*ctx->ptep)); > smp_store_release(ctx->ptep, new); > + return true; > } > > static bool stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt) > @@ -1001,7 +1064,7 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx, > return 0; > } > > - if (!stage2_try_break_pte(ctx, data->mmu)) > + if (!stage2_try_break_pte(ctx, data->mmu, new)) > return -EAGAIN; > > /* Perform CMOs before installation of the guest stage-2 PTE */ > @@ -1014,7 +1077,8 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx, > stage2_pte_executable(new)) > mm_ops->icache_inval_pou(kvm_pte_follow(new, mm_ops), granule); > > - stage2_make_pte(ctx, new); > + if (!stage2_make_pte(ctx, data->mmu, new)) > + return -EAGAIN; > > return 0; > } > @@ -1057,19 +1121,21 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx, > childp = mm_ops->zalloc_page(data->memcache); > if (!childp) > return -ENOMEM; > - > - if (!stage2_try_break_pte(ctx, data->mmu)) { > - mm_ops->put_page(childp); > - return -EAGAIN; > - } > - > /* > * If we've run into an existing block mapping then replace it with > * a table. Accesses beyond 'end' that fall within the new table > * will be mapped lazily. > */ > new = kvm_init_table_pte(childp, mm_ops); > - stage2_make_pte(ctx, new); > + if (!stage2_try_break_pte(ctx, data->mmu, new)) { > + mm_ops->put_page(childp); > + return -EAGAIN; > + } > + > + if (!stage2_make_pte(ctx, data->mmu, new)) { > + mm_ops->put_page(childp); > + return -EAGAIN; > + } > > return 0; > } > @@ -1549,18 +1615,21 @@ static int stage2_split_walker(const struct kvm_pgtable_visit_ctx *ctx, > if (IS_ERR(childp)) > return PTR_ERR(childp); > > - if (!stage2_try_break_pte(ctx, mmu)) { > - kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level); > - return -EAGAIN; > - } > - > /* > * Note, the contents of the page table are guaranteed to be made > * visible before the new PTE is assigned because stage2_make_pte() > * writes the PTE using smp_store_release(). > */ > new = kvm_init_table_pte(childp, mm_ops); > - stage2_make_pte(ctx, new); > + if (!stage2_try_break_pte(ctx, mmu, new)) { > + kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level); > + return -EAGAIN; > + } > + > + if (!stage2_make_pte(ctx, mmu, new)) { > + kvm_pgtable_stage2_free_unlinked(mm_ops, childp, level); > + return -EAGAIN; > + } > return 0; > } > > -- > 2.55.0.979.g7e5102b832-goog >