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Peter Anvin" , linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH v6 6/8] dma: swiotlb: Centralize memory-encryption pool sizing Message-ID: References: <20260924060756.1325156-1-aneesh.kumar@kernel.org> <20260924060756.1325156-7-aneesh.kumar@kernel.org> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Oct 07, 2026 at 11:43:06AM +0100, Will Deacon wrote: > On Wed, Oct 07, 2026 at 11:04:05AM +0100, Catalin Marinas wrote: > > On Tue, Oct 06, 2026 at 10:49:17PM +0100, Will Deacon wrote: > > > On Thu, Sep 24, 2026 at 11:37:54AM +0530, Aneesh Kumar K.V (Arm) wrote: > > > > @@ -496,7 +516,8 @@ swiotlb_select_pool_policy(unsigned int flags) > > > > if (swiotlb_force_disable) > > > > return SWIOTLB_POOL_NONE; > > > > > > > > - if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) > > > > + if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && > > > > + !restricted_dma_pool_present) > > > > return SWIOTLB_POOL_CC_GUEST; > > > > > > I think this check on the restricted DMA pool is too general -- the pool > > > could be tied to a specific DMA-capable peripheral and so treating its > > > presence as a global property isn't right. [...] > > That said, such heuristics should have been a separate patch to make it > > easier to review/drop. > > I think the only right way to get a semi-accurate heuristic is to take > into account the set of dma-capable devices that will use the swiotlb > pool, but that's fiddly and should probably be tackled as a separate > series. Maybe a simpler hack in that direction would be to take the > SWIOTLB_POOL_CC_GUEST if _any_ device is going to use swiotlb? You'll > run into the usual problem of not being able to tell if a device is > DMA-capable or not, but you could probably look for a global restricted > DMA pool and, if that doesn't exist, check for per-device restricted pools > on dma-coherent devices (since restricted DMA isn't supported by ACPI) as > a reasonable approximation. Yes, I think that's reasonable. We'd need to drive this from the arch code or, as you suggested, drivers/virt/coco rather than generic heuristics in the swiotlb code. -- Catalin