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[34.34.182.0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4aa820492cesm377375025e9.13.2026.11.02.06.22.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Nov 2026 06:22:26 -0800 (PST) Date: Mon, 2 Nov 2026 14:22:23 +0000 From: Vincent Donnefort To: Fuad Tabba Cc: maz@kernel.org, oupton@kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, kernel-team@android.com, qperret@google.com, weilin.chang@arm.com, tabba@google.com Subject: Re: [PATCH v2 22/22] KVM: arm64: Stage-2 huge mappings for protected VMs Message-ID: References: <20260911135053.146435-1-vdonnefort@google.com> <20260911135053.146435-23-vdonnefort@google.com> <20261029152959.3552746-1-fuad.tabba@linux.dev> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261102_062230_682274_B3E75768 X-CRM114-Status: GOOD ( 35.43 ) X-Spam-Score: -17.6 (-----------------) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-18.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIMWL_WL_MED,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Thu, Oct 29, 2026 at 04:14:10PM +0000, Vincent Donnefort wrote: > On Thu, Oct 29, 2026 at 03:29:59PM +0000, Fuad Tabba wrote: > > Hi Vincent, > > > > On Fri, 11 Sep 2026 14:50:53 +0100, Vincent Donnefort wrote: > > [...] > > > diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c > > [...] > > > @@ -513,17 +512,23 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size, > > [...] > > > + /* > > > + * If a huge mapping overlaps an existing PAGE_SIZE one, > > > + * then the VMM has played games with the stage-1. Abort. > > > + */ > > > + if (WARN_ON_ONCE(mapping && mapping->nr_pages == 1 && size > PAGE_SIZE)) > > > + return -EFAULT; > > > > A guest can hit this on its own, without the VMM touching its stage-1. > > I ran into it while porting your unposted v3 to android18-7.3, and v3 > > has the same race. A pVM shares one page of a 2M block, and another > > vCPU touches the block during the split's break-before-make. The host > > sizes that fault at 2M from the THP, but under the mmu_lock it finds > > the split's PAGE_SIZE mappings. > > Nice catch! > > > > > I made the following change in the android18-7.3 port. It maps just > > the faulting page, since retrying alone re-faults forever if the VMM > > puts a fresh THP behind a partly mapped block. If you're happy with it, > > can you fold it into v3? > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > index 70b002eb17e05..72fc2577f51cd 100644 > > --- a/arch/arm64/kvm/mmu.c > > +++ b/arch/arm64/kvm/mmu.c > > @@ -1921,6 +1921,7 @@ static int protected_pkvm_mem_abort(const struct kvm_s2_fault_desc *s2fd) > > struct mm_struct *mm = current->mm; > > struct kvm *kvm = vcpu->kvm; > > void *hyp_memcache; > > + u64 size; > > int ret; > > > > hyp_memcache = get_mmu_memcache(vcpu); > > @@ -1936,13 +1937,21 @@ static int protected_pkvm_mem_abort(const struct kvm_s2_fault_desc *s2fd) > > if (ret) > > goto unpin; > > > > + size = s2vi.vma_pagesize; > > kvm_fault_lock(kvm); > > if (mmu_invalidate_retry(kvm, s2vi.mmu_seq)) { > > ret = -EAGAIN; > > } else { > > - ret = pkvm_pgtable_stage2_map(pgt, gfn_to_gpa(s2vi.gfn), s2vi.vma_pagesize, > > + ret = pkvm_pgtable_stage2_map(pgt, gfn_to_gpa(s2vi.gfn), size, > > __pfn_to_phys(s2vi.pfn), KVM_PGTABLE_PROT_RWX, > > hyp_memcache, 0); > > + /* The block may hold PAGE_SIZE mappings from a split: map the page alone. */ > > + if (ret == -EAGAIN && size > PAGE_SIZE) { > > + size = PAGE_SIZE; > > + ret = pkvm_pgtable_stage2_map(pgt, ALIGN_DOWN(s2fd->fault_ipa, size), size, > > + page_to_phys(s2vi.page), KVM_PGTABLE_PROT_RWX, > > + hyp_memcache, 0); > > + } > > I do not believe the retry does anything, this would just hit the "if (mapping)" > path in pkvm_pgtable_stage2_map(). > > And in any case, the split will remap the entire range at PTE-level. This is > just transient and it is safe to return EAGAIN. > > To keep the return -EFAULT, I could eventually mark the mapping as "split in > progress". I convinced myself the retry is actually interesting for the case where the VMM is playing with the stage-1 mappings. The non-protected case supports it, so there's no reason for the protected one to not. And actually it does support directly into pkvm_pgtable_stage2_map(). So I believe that's where any sort of retry should happen. So I will try to refactor this path to avoid discrepancy between protected and non-protected. -- Vincent > > -- > Vincent > > > } > > kvm_fault_unlock(kvm); > > if (ret) { > > @@ -1953,6 +1962,9 @@ static int protected_pkvm_mem_abort(const struct kvm_s2_fault_desc *s2fd) > > goto unpin; > > } > > > > + if (size != s2vi.vma_pagesize) > > + account_locked_vm(mm, (s2vi.vma_pagesize - size) / PAGE_SIZE, false); > > + > > return 0; > > unpin: > > unpin_user_page(s2vi.page); > > diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c > > index 8a74df9f9fec3..6b5df4595f6a6 100644 > > --- a/arch/arm64/kvm/pkvm.c > > +++ b/arch/arm64/kvm/pkvm.c > > @@ -650,14 +650,8 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size, > > return -EINVAL; > > > > /* > > - * If a huge mapping overlaps an existing PAGE_SIZE one, > > - * then the VMM has played games with the stage-1. Abort. > > - */ > > - if (WARN_ON_ONCE(mapping && mapping->nr_pages == 1 && size > PAGE_SIZE)) > > - return -EFAULT; > > - > > - /* > > - * We either raced with another vCPU or the guest PTE > > + * We either raced with another vCPU, a block overlaps the > > + * PAGE_SIZE mappings of a guest-driven split, or the guest PTE > > * has been poisoned by an erroneous host access. > > */ > > if (mapping) { > > > > Cheers, > > /fuad >