Re: Purpose of translate MSI interrupt into INTx for guest passthrough

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Dropping Qing He as this address bounces.

On Tue, Jan 12, 2021 at 03:10:57PM +0100, Roger Pau Monné wrote:
> Hello,
> 
> While trying to do some cleanup of the Xen interrupt support for pci
> pass though I came across the MSI to INTx translation that Xen is in
> theory capable of performing (ie: use a physical MSI interrupt source
> and inject that as an INTx to a guest).
> 
> AFAICT such functionality is not wired up to the toolstack, so it's
> hard to tell what's the indented purpose, or whether it has seen any
> usage.

So apparently it is wired up to the toolstack for qemu-traditional,
albeit it's disabled by default. There's some documentation in
xl-pci-configuration:

"When enabled, MSI-INTx translation will always enable MSI on the PCI
device regardless of whether the guest uses INTx or MSI."

So the main purpose seem to be to always use the MSI interrupt source
regardless of whether the guest is using INTx or MSI. Maybe the
purpose was to workaround some bugs when using INTx? Or buggy devices
with INTx interrupts?

qemu-upstream doesn't seem to support it anymore, so I would still
like to remove it if we get consensus.

Roger