Re: [Xen-devel] [PATCH v5 5/6] xen/x86: add PHYSDEVOP_msi_control

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

On Fri, Jul 19, 2019 at 08:04:45AM +0000, Jan Beulich wrote:
> On 18.07.2019 18:52, Roger Pau Monné  wrote:
> > On Thu, Jul 18, 2019 at 03:17:27PM +0000, Jan Beulich wrote:
> >> On 18.07.2019 15:46, Roger Pau Monné  wrote:
> >>> In fact I don't think INTx should be enabled when MSI(-X) is disabled,
> >>> QEMU already traps writes to the command register, and it will manage
> >>> INTx enabling/disabling by itself. I think the only check required is
> >>> that MSI(-X) cannot be enabled if INTx is also enabled. In the same
> >>> way both MSI caspabilities cannot be enabled simultaneously. The
> >>> function should not explicitly disable any of the other capabilities,
> >>> and just return -EBUSY if the caller attempts for example to enable
> >>> MSI while INTx or MSI-X is enabled.
> >>
> >> You do realize that pci_intx() only ever gets called for Xen
> >> internally used interrupts, i.e. mainly the serial console one?
> > 
> > You will have to bear with me because I'm not sure I understand why
> > it does matter. Do you mean to point out that dom0 is the one in full
> > control of INTx, and thus Xen shouldn't care of whether INTx and
> > MSI(-X) are enabled at the same time?
> > 
> > I still think that at least a warning should be printed if a caller
> > tries to enable MSI(-X) while INTx is also enabled, but unless there's
> > a reason to have both MSI(-X) and INTx enabled at the same time (maybe
> > a quirk for some hardware issue?) it shouldn't be allowed on this new
> > interface.
> 
> I don't mind improvements to the current situation (i.e. such a
> warning may indeed make sense); I merely stated how things currently
> are. INTx treatment was completely left aside when MSI support was
> introduced into Xen.

In order to give Marek a more concise reply, would you agree to return
-EBUSY (or some error code) and print a warning message if the caller
attempts to enable MSI(-X) while INTx is also enabled?

The same would apply to enabling both MSI(-X) capabilities at the
same time.

Thanks, Roger.

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