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[34.124.234.44]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-83965a3e3ecsm20960916b3a.19.2026.05.11.06.21.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2026 06:21:37 -0700 (PDT) Date: Mon, 11 May 2026 13:21:31 +0000 From: Pranjal Shrivastava To: Robin Murphy Cc: Jason Gunthorpe , Nicolin Chen , Will Deacon , Joerg Roedel , Jean-Philippe Brucker , Catalin Marinas , =?utf-8?Q?Miko=C5=82aj?= Lenczewski , linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] iommu/arm-smmu-v3-sva: Enable Hardware Access and Hardware Dirty bits Message-ID: References: <20260503135413.1108138-1-nicolinc@nvidia.com> <20260508123550.GB9254@nvidia.com> <4e129891-2f52-4bac-8e33-1fdde42fd29a@arm.com> Precedence: bulk X-Mailing-List: iommu@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: <4e129891-2f52-4bac-8e33-1fdde42fd29a@arm.com> On Fri, May 08, 2026 at 03:24:32PM +0100, Robin Murphy wrote: > On 2026-05-08 2:57 pm, Pranjal Shrivastava wrote: > > On Fri, May 08, 2026 at 02:31:11PM +0100, Robin Murphy wrote: > > > On 2026-05-08 2:12 pm, Pranjal Shrivastava wrote: > > > > On Fri, May 08, 2026 at 09:35:50AM -0300, Jason Gunthorpe wrote: > > > > > On Thu, May 07, 2026 at 10:30:14PM +0000, Pranjal Shrivastava wrote: > > > > > > > @@ -92,6 +92,16 @@ void arm_smmu_make_sva_cd(struct arm_smmu_cd *target, > > > > > > > target->data[1] = cpu_to_le64(virt_to_phys(mm->pgd) & > > > > > > > CTXDESC_CD_1_TTB0_MASK); > > > > > > > + > > > > > > > + /* > > > > > > > + * Enable Hardware Access and Dirty updates (DBM) if supported. > > > > > > > + * This is safe to enable by default, as PTE_WRITE and PTE_DBM > > > > > > > + * share the same bit. > > > > > > > + */ > > > > > > > + if (master->smmu->features & ARM_SMMU_FEAT_HA) > > > > > > > + target->data[0] |= cpu_to_le64(CTXDESC_CD_0_TCR_HA); > > > > > > > + if (master->smmu->features & ARM_SMMU_FEAT_HD) > > > > > > > + target->data[0] |= cpu_to_le64(CTXDESC_CD_0_TCR_HD); > > > > > > > > > > > > IIUC, we should be setting these if IO_PGTABLE_QUIRK_ARM_HD is present? > > > > > > > > > > SVA does not use IO_PGTABLE at all, and it directly constructs its own > > > > > CD. > > > > > > > > > > No relation between those two flows. > > > > > > > > I understand that but I mean we need to know if the system supports > > > > HTTU ? Like for SMMU we use the IO_PGTABLE_QUIRK, shouldn't we be > > > > checking if the CPU's tables support HTTU? > > > > > > > > Are we assuming that if the SMMU IDR presents HTTU capability the MMU > > > > would also have it? I think an unconditional enablement is risky as we > > > > may not have system-wide HTTU support. > > > > > > > > If we look at arm_smmu_master_sva_supported, the driver already > > > > maintains a strict agreement between the CPU and SMMU for SVA. > > > > It checks sanitized CPU ID registers for things like PARANGE & ASIDBITS, > > > > and it uses system_supports_bbml2_noabort() to decide whether to enable > > > > FEAT_BBML2. > > > > > > > > Shouldn't we follow this exact same pattern for HTTU ? > > > > We should probably be checking cpu_has_hw_af() (from asm/cpufeature.h) > > > > in the SVA support check or here if we wanna enable HTTU. > > > > > > It might make sense to depend on CONFIG_ARM64_HW_AFDBM - when that is > > > enabled, then IIRC we already expect to cope with some CPUs not supporting > > > hardware updates, so it should still be fine for an SMMU to make them even > > > if no CPU does. However, if it's disabled then I'm not sure if missing > > > access flag faults (if SMMU HA silently sets them) might be an issue - for > > > dirty, we'd just never put down the Writeable-Clean permission so enabling > > > SMMU HD wouldn't do anything anyway. > > > > I see, so IIUC, you mean if IS_ENABLED(CONFIG_ARM64_HW_AFDBM) but CPU > > doesn't enable HTTU, it is perfectly safe to let the SMMU do HTT updates, > > Since the fault handlers are already expecting HW-triggered updates? > > > > Which means our check would be something like: > > > > if (IS_ENABLED(CONFIG_ARM64_HW_AFDBM) { > > if (smmu->features & FEAT_HA) > > ... > > } > > > > instead of cpu_has_hw_af()? > > Hmm, looking closer, cpu_has_hw_af() is the thing which actually influences > mm behaviour (via arch_has_hw_pte_young and arch_wants_old_prefaulted_pte), > and that can still be false at runtime if ARM64_HW_AFDBM is enabled but any > CPU doesn't support HAFDBS, so perhaps you were right the first time :) > Yea, I believe the cpu_has_hw_af() is the right gate. > Although AFAICS from __cpu_setup(), ARM64_HW_AFDBM will still > unconditionally enable TCR_EL1.HA on CPUs which do support it, so maybe it > is OK anyway? > I believe cpu_has_hw_af() is still the safer gate for SVA. While individual cores might turn on their local HA support, cpu_has_hw_af() represents the sanitized system view. In mismatched systems (where some cores support HAFDBS and others don't), cpu_has_hw_af() will be false & mm shall default to software-managed AF/ Dirty for consistency across all threads. Enabling HTTU in the SMMU while the kernel mm is in 'SW-Managed' mode could cause the SMMU to silently flip bits that the kernel is expecting to handle via faults, leading to a mismatch. Thanks, Praan