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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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

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