Re: [Qemu-devel] [Xen-devel] [PATCH 5/6] xen-pt: Hide MSI-X from xen stubdoms

[Marek Marczykowski-Górecki]

On Tue, Mar 12, 2019 at 09:58:56AM -0400, Jason Andryuk wrote:
> On Tue, Mar 12, 2019 at 8:38 AM Marek Marczykowski-Górecki
> <address@hidden> wrote:
> >
> > On Tue, Mar 12, 2019 at 01:04:19PM +0100, Roger Pau Monné wrote:
> > > On Mon, Mar 11, 2019 at 02:02:15PM -0400, Jason Andryuk wrote:
> > > > MSI-X is not supported in Xen stubdoms, so it must be disabled.  Use the
> > > > existing xen_pt_hide_dev_cap to hide when running under -xen-stubdom.
> > >
> > > I'm afraid this requires some more context. What's the actual issue
> > > that prevents MSI-X from working?
> >
> > At least missing "Fix PCI passthrough for HVM with stubdomain" series,
> > but that's mostly on Xen side (+ one change how QEMU enable MSI-X in
> > config space).
> > Some of it can be worked around by enabling permissive mode. Jason, did
> > you had a chance to test it with any MSI-X device?
> > I'm not aware of anything thing particular that breaks MSI-X but not
> > MSI. Besides much less devices lying around to test MSI-X...
> 
> OpenXT and Qubes have used a compile time patch that disabled MSI-X
> for a long time.  The OpenXT patch description doesn't help:
> """
> Currently we do not support MSI-X setup for PCI devices passed through.
> 
> Although the specification mentions that PCI-e devices might implement only
> MSI-X there is not a lot of those and mostly none that we have encountered 
> yet.
> Considering that, we force devices to use MSI by hiding the MSI-X capability.
> """
> 
> To be honest, I didn't question the reasoning and just made the
> compile-time disabling into a runtime disabling.
> 
> I tested with a NEC uPD720200 XHCI controller supporting MSI-X.  There
> was an error related to setting up MSI-X when I failed to pass the
> "-xen-stubdom" flag.  I can pull that log when I get back to the
> machine.  With this patch, MSI-X was hidden in the guest, but dom0
> showed MSI-X present but unused.
> 
> Marek, is "Use xc_physdev_msi_set_enable for enabling MSI..." the QEMU
> patch you are refer to?  Do you think permissive mode would allow
> MSI-X to work without that patch?  I could test that out.

Yes, this one. Permissive mode should work around it.
There is also another patch about IRQ permission, but I believe you
already have it in OpenXT.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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