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Tsirkin" To: Zhu Lingshan Cc: David Woodhouse , virtio-comment@lists.linux.dev, hch@infradead.org, Claire Chang , linux-devicetree , Rob Herring , =?iso-8859-1?Q?J=F6rg?= Roedel , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, graf@amazon.de Subject: Re: [RFC PATCH 3/3] transport-pci: Add SWIOTLB bounce buffer capability Message-ID: <20250403033718-mutt-send-email-mst@kernel.org> References: <20250402112410.2086892-1-dwmw2@infradead.org> <20250402112410.2086892-4-dwmw2@infradead.org> <20250403032152-mutt-send-email-mst@kernel.org> <9635c502-3635-4875-ae1f-53b4b7aed855@amd.com> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <9635c502-3635-4875-ae1f-53b4b7aed855@amd.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: T_vZZswKxvLZUQVRr_i8E6_aNil8-_Ihtma2dn4iaj4_1743665854 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Thu, Apr 03, 2025 at 03:36:04PM +0800, Zhu Lingshan wrote: > On 4/3/2025 3:27 PM, Michael S. 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 Hmm which TLB? I don't get it. > > 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 ... > >