[virtio-dev] Re: spec inconsistency: Device Configuration Interrupt bit in ISR status

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

On Mon, Jan 17, 2022 at 02:15:55PM +0800, Jason Wang wrote:
> 
> 在 2022/1/14 下午9:39, Michael S. Tsirkin 写道:
> > The spec says (v1.1 4.1.4.5 ISR status capability):
> > 
> > The VIRTIO_PCI_CAP_ISR_CFG capability refers to at least a single byte, which contains the 8­bit ISR
> > status field to be used for INT#x interrupt handling.
> > 
> > and
> > 
> > to avoid an extra access, simply reading this register resets it to 0 and causes the device to de­assert the
> > interrupt.
> > In this way, driver read of ISR status causes the device to de­assert an interrupt.
> > 
> > See sections 4.1.5.3 and 4.1.5.4 for how this is used.
> > 
> > and in 4.1.5.4 Notification of Device Configuration Changes
> > 
> > it says:
> > 
> > • If MSI­X capability is disabled:
> > 1. Set the second lower bit of the ISR Status field for the device.
> > 2. Send the appropriate PCI interrupt for the device.
> > 
> > If MSI­X capability is enabled:
> > 1. If config_msix_vector is not NO_VECTOR, request the appropriate MSI­X interrupt message for
> > the device, config_msix_vector sets the MSI­X Table entry number.
> > 
> > all of the above make it looks like VIRTIO_PCI_CAP_ISR_CFG capability is
> > unused with MSIX.
> > 
> > This was actually the way the spec was understood by
> > Zhu Lingshan from Intel (Cc'd).
> > 
> > However, looking at the conformance statements, one finds out this is
> > not the case:
> > 
> > 4.1.4.5.1 Device Requirements: ISR status capability
> > 
> > 
> > The device MUST present at least one VIRTIO_PCI_CAP_ISR_CFG capability.
> > The device MUST set the Device Configuration Interrupt bit in ISR status before sending a device configu­
> > ration change notification to the driver.
> > If MSI­X capability is disabled, the device MUST set the Queue Interrupt bit in ISR status before sending a
> > virtqueue notification to the driver.
> > 
> > which implies that the Device Configuration Interrupt bit is set unconditionally.
> > 
> > 
> > 
> > It is unfortunate that it does not copy this requirement in more places,
> > and that the non-conformance text is incomplete and does not
> > mention the MSI-X usage at all.
> > 
> > I propose to extend 4.1.4.5 ISR status capability and
> > 4.1.5.4 Notification of Device Configuration Changes
> > to mention the MSI use.
> 
> 
> I wonder do we want
> 
> 1) mandate ISR bit
> 
> or
> 
> 2) remove the ISR bit set for MSI mode?
> 
> 1) is the current Qemu behavior but seems a little bit contradict with the
> goal of MSI.
> 
> Thanks
> 
> 

I think we want the ISR bit since otherwise we need to go read
a ton of config space fields to check whether anything changed.

-- 
MST


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