Re: MSI for Virtio PCI transport

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 25, 2024 at 08:00:45AM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, June 25, 2024 1:25 PM
> > To: Parav Pandit <parav@nvidia.com>
> > Cc: Manivannan Sadhasivam <manisadhasivam.linux@gmail.com>; 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