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

From: Jason Andryuk <jandryuk@gmail.com>
To: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
Cc: "James McKenzie" <james.mckenzie@bromium.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	qemu-devel@nongnu.org, "Paul Durrant" <paul.durrant@citrix.com>,
	"Anthony Perard" <anthony.perard@citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 5/6] xen-pt: Hide MSI-X from xen stubdoms
Date: Tue, 12 Mar 2019 09:58:56 -0400	[thread overview]
Message-ID: <CAKf6xpvVBDGCmbcgU0u39Sh5NjFRf95O3UQ=_DmD6uVb-vx-sA@mail.gmail.com> (raw)
In-Reply-To: <20190312123808.GG1208@mail-itl>

On Tue, Mar 12, 2019 at 8:38 AM Marek Marczykowski-Górecki
<marmarek@invisiblethingslab.com> 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.

Regards,
Jason

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