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[34.77.69.137]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f4635a9cesm64944f8f.14.2026.07.13.08.14.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2026 08:14:59 -0700 (PDT) Date: Mon, 13 Jul 2026 16:14:55 +0100 From: Vincent Donnefort To: Mostafa Saleh Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, iommu@lists.linux.dev, catalin.marinas@arm.com, will@kernel.org, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, joro@8bytes.org, jean-philippe@linaro.org, jgg@ziepe.ca, mark.rutland@arm.com, qperret@google.com, tabba@google.com, sebastianene@google.com, keirf@google.com Subject: Re: [PATCH v6 08/25] KVM: arm64: iommu: Shadow host stage-2 page table Message-ID: References: <20260501111928.259252-1-smostafa@google.com> <20260501111928.259252-9-smostafa@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jul 13, 2026 at 02:00:31PM +0000, Mostafa Saleh wrote: > On Mon, Jul 13, 2026 at 02:24:19PM +0100, Vincent Donnefort wrote: > > On Fri, May 01, 2026 at 11:19:10AM +0000, Mostafa Saleh wrote: > > > Create a page-table for the IOMMU that shadows the host CPU stage-2 > > > to establish DMA isolation. > > > > > > An initial snapshot is created after the driver init, then > > > on every permission change a callback would be called for > > > the IOMMU driver to update the page table. > > > > > [...] > > > > + */ > > > + if (pte && !kvm_pte_valid(pte)) > > > + return 0; > > > + > > > + if (kvm_pte_valid(pte)) { > > > + prot = pkvm_to_iommu_prot(kvm_pgtable_stage2_pte_prot(pte)); > > > + /* If the range is mapped in a single PTE, it must be the same type.*/ > > > + if (!addr_is_memory(start)) > > > + prot |= IOMMU_MMIO; > > > + > > > + return kvm_iommu_ops->host_stage2_idmap(start, end, prot); > > > > Do we really need to do that when is_memory()? > > > > fix_host_ownership_walker() by calling host_stage2_idmap_locked() and > > host_stage2_set_owner_locked() should already handle the memory region. That > > would also get rid of kvm_idmap_initialized. > > > > So this one here could only take care of the MMIO? > > > > Overall we would have a common point of synchro which is > > fix_host_ownership_walker() after which the host ownership is ready for both > > CPU stage-2 and the IOMMU? > > > > I am not sure I understand, this is another empty page table, so we > have to walk all of the host CPU stage-2 page table to shadow it in the > IOMMU. if you are refering to the case where it handle zero ptes for > memory, I can drop that but it will not change much in this logic. In fixup_host_ownership() we already walk the hyp pgtable to know what needs to be map/unmapped from the host stage-2. Can't we rely on that for the IOMMU page-table as well? As of, fix_host_ownership() could handle the host stage-2 __and__ the iommu? > > > > + } > > > + > > > + /* In case of invalid PTE, we need to figure out which part of it is MMIO */ > > [...] > > > > #include > > > @@ -481,6 +482,14 @@ static int check_range_allowed_memory(u64 start, u64 end) > > > return 0; > > > } > > > > > > +u64 find_mem_range_from(u64 start, bool *is_memory) > > > +{ > > > + struct kvm_mem_range r; > > > + > > > + *is_memory = !!find_mem_range(start, &r); > > > + return r.end; > > > +} > > > + > > > static bool range_is_memory(u64 start, u64 end) > > > { > > > struct kvm_mem_range r; > > > @@ -577,8 +586,34 @@ int host_stage2_idmap_locked(phys_addr_t addr, u64 size, > > > > > > static void __host_update_page_state(phys_addr_t addr, u64 size, enum pkvm_page_state state) > > > > I would really split this. I know that this is convinient, but as the function > > says, it only update the page state so it shouldn't hide an update to the IOMMU. > > I mention a couple of alternatives in the commit message, I tried to > implement it differently which was harder to reason about as the calls > was scattered everywhere and any small refactor will possibly break it. > > Did you have a split in my mind? I open to rework it. Yes, I meant the option #2. > > > > > Beside, we have examples already in Android where we want to update the > > page-state but not the IOMMU, so it doesn't feel future-proof... > > > > > { > > > + enum pkvm_page_state old = get_host_state(hyp_phys_to_page(addr)); > > > + enum kvm_pgtable_prot prot = 0; > > > + > > > for_each_hyp_page(page, addr, size) > > > set_host_state(page, state); > > > + > > > + /* > > > + * Any transition to PKVM_NOPAGE, unmaps the page from the host > > > + * Any transition to PKVM_PAGE_SHARED_BORROWED, maps the page in the host > > > + * Any transition to PKVM_PAGE_SHARED_OWNED is ignored as page is already mapped. > > > + * Transitions to PKVM_PAGE_OWNED from anything but PKVM_NOPAGE are ignored. > > > + * Transitions to PKVM_PAGE_OWNED from PKVM_NOPAGE will map the page. > > > + */ > > > + if ((state == PKVM_PAGE_SHARED_OWNED) || > > > + ((state == PKVM_PAGE_OWNED) && (old != PKVM_NOPAGE))) > > > + return; > > > + > > > + if ((state == PKVM_PAGE_SHARED_BORROWED) || > > > + (state == PKVM_PAGE_OWNED)) > > > + prot = PKVM_HOST_MEM_PROT; > > > > ... and that would avoid that sort of things here. The caller decides if the IOMMU > > is updated or not. > > Typically, the IOMMU is updated if the CPU is. > > > > > And as the patch says, we "shadow" the host stage2. So probably modifying > > host_stage2_idmap and host_stage2_set_owner_metdata() sounds really a better > > approach. > > Initially, before the pKVM merge upstream I was doing something similar > as that only required one hook [1]. However, after rebasing I found that > would be too complicated and I have to add many more (as mentioned in > the commit message). But I can re-visit this approach in v7. What bit would be more complicated? As you've wrote in the commit message, in option#2 you only need two calls really: * host_stage2_set_owner_locked() * host_stage2_set_owner_metadata_locked(). This fits better the narative for the "shadow" page-table: when we map the host stage-2, we map the iommu and when we unmap the host stage-2, we unmap the iommu. > > [1] https://lore.kernel.org/all/20250819215156.2494305-11-smostafa@google.com/ > > Thanks, > Mostafa >