Re: [virtio-dev] Re: [PATCH v3 3/4] content: Introduce driver/device auxiliary notifications for MMIO

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

On Mon, Oct 31, 2022 at 01:24:30PM -0400, Stefan Hajnoczi wrote:
> On Mon, Oct 31, 2022 at 11:45:55AM -0400, Michael S. Tsirkin wrote:
> > On Thu, Oct 27, 2022 at 05:27:28PM -0400, Stefan Hajnoczi wrote:
> > > On Fri, Oct 07, 2022 at 05:56:42PM +0100, Usama Arif wrote:
> > > > This includes the additions to the corresponding device and driver
> > > > conformances.
> > > > 
> > > > Signed-off-by: Usama Arif <usama.arif@bytedance.com>
> > > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > 
> > > You can keep this. One thought:
> > > 
> > > I realized that virtio-mmio support won't be very useful until
> > > virtio-mmio gets MSI-X support because drivers currently cannot
> > > differentiate between device auxiliary notifications.
> > > 
> > > Stefan
> > 
> > 
> > The big issue with MSI is it actually needs a ton of registers
> > for interrupt rebalancing, masking etc and they need to be fast.
> > Maybe a status bit like pci has makes sense here.
> 
> This patch does add an ISR bit:
> 
> diff --git a/content.tex b/content.tex
> index 33362b7..8968fcd 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2049,6 +2049,8 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>          in at least one of the active virtual queues.
>        \item [Configuration Change Notification] - bit 1 - the interrupt was
>          asserted because the configuration of the device has changed.
> +      \item [Device-specific Driver Auxiliary Notification] - bit 2 - the interrupt was
> +        asserted because a device-specific event occurred to notify the driver.
> 
> The problem is that the driver doesn't know which device-specific event
> occurred, so the mechanism is less useful than the MSI-X approach where
> the driver knows exactly which (of potentially many) device-specific
> events occurred.
> 
> Stefan


Right I figured that out after sending this. So it's limited to
one event type.


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