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Tsirkin" To: Parav Pandit Cc: Manivannan Sadhasivam , "virtio-comment@lists.linux.dev" , "mie@igel.co.jp" Subject: Re: MSI for Virtio PCI transport Message-ID: <20240625040729-mutt-send-email-mst@kernel.org> References: <20240624161957.GB3179@thinkpad> <20240625054346.GA2642@thinkpad> <20240625035343-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: virtio-comment@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Jun 25, 2024 at 08:00:45AM +0000, Parav Pandit wrote: > > > From: Michael S. Tsirkin > > Sent: Tuesday, June 25, 2024 1:25 PM > > To: Parav Pandit > > Cc: Manivannan Sadhasivam ; virtio- > > comment@lists.linux.dev; mie@igel.co.jp > > Subject: Re: MSI for Virtio PCI transport > > > > On Tue, Jun 25, 2024 at 06:18:46AM +0000, Parav Pandit wrote: > > > > So MSI-X is clearly an optional feature which simple devices tend to ignore. > > > > But if both are supported, then obviously Virtio will make use of > > > > MSI-X, but that's not the case here. > > > > > > > If both are supported, and required scale by the driver is <=32, driver can > > choose MSI due to its lightweight nature. > > > > Unlikely, MSI vectors are tricky to mask and this is a problem for interrupt > > balancing. So MSIX is better for performance even if the # of vectors is low. > Masking to my knowledge is not used by MSIX. There's a mask bit per vector, yes. > Didn't follow how MSIX helps with performance. Linux uses mask/change/unmask to balance interrupts between CPUs. *That* is important for performance. > The benefit of MSI is it does not need to store addr+data pair per vector. The addr/data thing wasn't invented just to make hardware costs go up. > > > > -- > > MST