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Tsirkin" , David Woodhouse Cc: virtio-comment@lists.linux.dev, hch@infradead.org, Claire Chang , linux-devicetree , Rob Herring , =?UTF-8?Q?J=C3=B6rg_Roedel?= , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, graf@amazon.de References: <20250402112410.2086892-1-dwmw2@infradead.org> <20250402112410.2086892-4-dwmw2@infradead.org> <20250403032152-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Zhu Lingshan In-Reply-To: <20250403032152-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI2P153CA0022.APCP153.PROD.OUTLOOK.COM (2603:1096:4:190::8) To SN7PR12MB7835.namprd12.prod.outlook.com (2603:10b6:806:328::22) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB7835:EE_|CH3PR12MB9395:EE_ X-MS-Office365-Filtering-Correlation-Id: 8634a022-0b66-453e-6891-08dd7282357a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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Tsirkin wrote: > On Wed, Apr 02, 2025 at 12:04:47PM +0100, David Woodhouse wrote: >> From: David Woodhouse >> >> Add a VIRTIO_PCI_CAP_SWIOTLB capability which advertises a SWIOTLB bounce >> buffer similar to the existing `restricted-dma-pool` device-tree feature. >> >> The difference is that this is per-device; each device needs to have its >> own. Perhaps we should add a UUID to the capability, and have a way for >> a device to not *provide* its own buffer, but just to reference the UUID >> of a buffer elsewhere? >> >> Signed-off-by: David Woodhouse >> --- >> transport-pci.tex | 33 +++++++++++++++++++++++++++++++++ >> 1 file changed, 33 insertions(+) >> >> diff --git a/transport-pci.tex b/transport-pci.tex >> index a5c6719..23e0d57 100644 >> --- a/transport-pci.tex >> +++ b/transport-pci.tex >> @@ -129,6 +129,7 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option >> \item ISR Status >> \item Device-specific configuration (optional) >> \item PCI configuration access >> +\item SWIOTLB bounce buffer >> \end{itemize} >> >> Each structure can be mapped by a Base Address register (BAR) belonging to >> @@ -188,6 +189,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option >> #define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8 >> /* Vendor-specific data */ >> #define VIRTIO_PCI_CAP_VENDOR_CFG 9 >> +/* Software IOTLB bounce buffer */ >> +#define VIRTIO_PCI_CAP_SWIOTLB 10 >> \end{lstlisting} >> >> Any other value is reserved for future use. >> @@ -744,6 +747,36 @@ \subsubsection{Vendor data capability}\label{sec:Virtio >> The driver MUST qualify the \field{vendor_id} before >> interpreting or writing into the Vendor data capability. >> >> +\subsubsection{Software IOTLB bounce buffer capability}\label{sec:Virtio >> +Transport Options / Virtio Over PCI Bus / PCI Device Layout / >> +Software IOTLB bounce buffer capability} >> + >> +The optional Software IOTLB bounce buffer capability allows the >> +device to provide a memory region which can be used by the driver >> +driver for bounce buffering. This allows a device on the PCI >> +transport to operate without DMA access to system memory addresses. >> + >> +The Software IOTLB region is referenced by the >> +VIRTIO_PCI_CAP_SWIOTLB capability. Bus addresses within the referenced >> +range are not subject to the requirements of the VIRTIO_F_ORDER_PLATFORM >> +capability, if negotiated. >> + >> +\devicenormative{\paragraph}{Software IOTLB bounce buffer capability}{Virtio >> +Transport Options / Virtio Over PCI Bus / PCI Device Layout / >> +Software IOTLB bounce buffer capability} >> + >> +Devices which present the Software IOTLB bounce buffer capability >> +SHOULD also offer the VIRTIO_F_SWIOTLB feature. >> + >> +\drivernormative{\paragraph}{Software IOTLB bounce buffer capability}{Virtio >> +Transport Options / Virtio Over PCI Bus / PCI Device Layout / >> +Software IOTLB bounce buffer capability} >> + >> +The driver SHOULD use the offered buffer in preference to passing system >> +memory addresses to the device. If the driver accepts the VIRTIO_F_SWIOTLB >> +feature, then the driver MUST use the offered buffer and never pass system >> +memory addresses to the device. >> + >> \subsubsection{PCI configuration access capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / PCI configuration access capability} >> >> The VIRTIO_PCI_CAP_PCI_CFG capability >> -- >> 2.49.0 >> > > > So on the PCI option. The normal mapping (ioremap) for BAR is uncached. If done > like this, performance will suffer. But if you do normal WB, since device and this even possibly can cause TLB thrashing.... which is a worse case. Thanks Zhu Lingshan > accesses do not go on the bus, they do not get synchronized with driver > writes and there's really no way to synchronize them. > > First, this needs to be addressed. > > In this age of accelerators for everything, building pci based > interfaces that can't be efficiently accelerated seems shortsighted ... >