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[2a00:1028:83b8:1e7a:3010:3bd6:8521:caf1]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47d7443c04csm28265855e9.1.2026.01.05.01.50.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Jan 2026 01:50:20 -0800 (PST) Date: Mon, 5 Jan 2026 10:50:18 +0100 From: Petr Tesarik To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Cong Wang , Jonathan Corbet , Olivia Mackall , Herbert Xu , Jason Wang , Paolo Bonzini , Stefan Hajnoczi , Eugenio =?UTF-8?B?UMOpcmV6?= , "James E.J. Bottomley" , "Martin K. Petersen" , Gerd Hoffmann , Xuan Zhuo , Marek Szyprowski , Robin Murphy , Stefano Garzarella , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Leon Romanovsky , Jason Gunthorpe , Bartosz Golaszewski , linux-doc@vger.kernel.org, linux-crypto@vger.kernel.org, virtualization@lists.linux.dev, linux-scsi@vger.kernel.org, iommu@lists.linux.dev, kvm@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH v2 03/15] dma-mapping: add DMA_ATTR_CPU_CACHE_CLEAN Message-ID: <20260105105018.732dd009@mordecai> In-Reply-To: <2d5d091f9d84b68ea96abd545b365dd1d00bbf48.1767601130.git.mst@redhat.com> References: <2d5d091f9d84b68ea96abd545b365dd1d00bbf48.1767601130.git.mst@redhat.com> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-suse-linux-gnu) Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 5 Jan 2026 03:23:01 -0500 "Michael S. Tsirkin" wrote: > When multiple small DMA_FROM_DEVICE or DMA_BIDIRECTIONAL buffers share a > cacheline, and DMA_API_DEBUG is enabled, we get this warning: > cacheline tracking EEXIST, overlapping mappings aren't supported. > > This is because when one of the mappings is removed, while another one > is active, CPU might write into the buffer. > > Add an attribute for the driver to promise not to do this, making the > overlapping safe, and suppressing the warning. > > Signed-off-by: Michael S. Tsirkin LGTM. I'm not formally a reviewer, but FWIW: Reviewed-by: Petr Tesarik > --- > include/linux/dma-mapping.h | 7 +++++++ > kernel/dma/debug.c | 3 ++- > 2 files changed, 9 insertions(+), 1 deletion(-) > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 29ad2ce700f0..29973baa0581 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -79,6 +79,13 @@ > */ > #define DMA_ATTR_MMIO (1UL << 10) > > +/* > + * DMA_ATTR_CPU_CACHE_CLEAN: Indicates the CPU will not dirty any cacheline > + * overlapping this buffer while it is mapped for DMA. All mappings sharing > + * a cacheline must have this attribute for this to be considered safe. > + */ > +#define DMA_ATTR_CPU_CACHE_CLEAN (1UL << 11) > + > /* > * A dma_addr_t can hold any valid DMA or bus address for the platform. It can > * be given to a device to use as a DMA source or target. It is specific to a > diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c > index 138ede653de4..7e66d863d573 100644 > --- a/kernel/dma/debug.c > +++ b/kernel/dma/debug.c > @@ -595,7 +595,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs) > if (rc == -ENOMEM) { > pr_err_once("cacheline tracking ENOMEM, dma-debug disabled\n"); > global_disable = true; > - } else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC) && > + } else if (rc == -EEXIST && > + !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_CPU_CACHE_CLEAN)) && > !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && > is_swiotlb_active(entry->dev))) { > err_printk(entry->dev, entry,