Date:   Wed, 28 Jun 2023 18:00:16 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
cc:     Petr Pavlu <petr.pavlu@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jgross@...e.com" <jgross@...e.com>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>
Subject: Re: [PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0

On Wed, 21 Jun 2023, Oleksandr Tyshchenko wrote:
> On 21.06.23 16:12, Petr Pavlu wrote:
> 
> 
> Hello Petr
> 
> 
> > When attempting to run Xen on a QEMU/KVM virtual machine with virtio
> > devices (all x86_64), dom0 tries to establish a grant for itself which
> > eventually results in a hang during the boot.
> > 
> > The backtrace looks as follows, the while loop in __send_control_msg()
> > makes no progress:
> > 
> >    #0  virtqueue_get_buf_ctx (_vq=_vq@...ry=0xffff8880074a8400, len=len@...ry=0xffffc90000413c94, ctx=ctx@...ry=0x0 <fixed_percpu_data>) at ../drivers/virtio/virtio_ring.c:2326
> >    #1  0xffffffff817086b7 in virtqueue_get_buf (_vq=_vq@...ry=0xffff8880074a8400, len=len@...ry=0xffffc90000413c94) at ../drivers/virtio/virtio_ring.c:2333
> >    #2  0xffffffff8175f6b2 in __send_control_msg (portdev=<optimized out>, port_id=0xffffffff, event=0x0, value=0x1) at ../drivers/char/virtio_console.c:562
> >    #3  0xffffffff8175f6ee in __send_control_msg (portdev=<optimized out>, port_id=<optimized out>, event=<optimized out>, value=<optimized out>) at ../drivers/char/virtio_console.c:569
> >    #4  0xffffffff817618b1 in virtcons_probe (vdev=0xffff88800585e800) at ../drivers/char/virtio_console.c:2098
> >    #5  0xffffffff81707117 in virtio_dev_probe (_d=0xffff88800585e810) at ../drivers/virtio/virtio.c:305
> >    #6  0xffffffff8198e348 in call_driver_probe (drv=0xffffffff82be40c0 <virtio_console>, drv=0xffffffff82be40c0 <virtio_console>, dev=0xffff88800585e810) at ../drivers/base/dd.c:579
> >    #7  really_probe (dev=dev@...ry=0xffff88800585e810, drv=drv@...ry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:658
> >    #8  0xffffffff8198e58f in __driver_probe_device (drv=drv@...ry=0xffffffff82be40c0 <virtio_console>, dev=dev@...ry=0xffff88800585e810) at ../drivers/base/dd.c:800
> >    #9  0xffffffff8198e65a in driver_probe_device (drv=drv@...ry=0xffffffff82be40c0 <virtio_console>, dev=dev@...ry=0xffff88800585e810) at ../drivers/base/dd.c:830
> >    #10 0xffffffff8198e832 in __driver_attach (dev=0xffff88800585e810, data=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:1216
> >    #11 0xffffffff8198bfb2 in bus_for_each_dev (bus=<optimized out>, start=start@...ry=0x0 <fixed_percpu_data>, data=data@...ry=0xffffffff82be40c0 <virtio_console>,
> >        fn=fn@...ry=0xffffffff8198e7b0 <__driver_attach>) at ../drivers/base/bus.c:368
> >    #12 0xffffffff8198db65 in driver_attach (drv=drv@...ry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:1233
> >    #13 0xffffffff8198d207 in bus_add_driver (drv=drv@...ry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/bus.c:673
> >    #14 0xffffffff8198f550 in driver_register (drv=drv@...ry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/driver.c:246
> >    #15 0xffffffff81706b47 in register_virtio_driver (driver=driver@...ry=0xffffffff82be40c0 <virtio_console>) at ../drivers/virtio/virtio.c:357
> >    #16 0xffffffff832cd34b in virtio_console_init () at ../drivers/char/virtio_console.c:2258
> >    #17 0xffffffff8100105c in do_one_initcall (fn=0xffffffff832cd2e0 <virtio_console_init>) at ../init/main.c:1246
> >    #18 0xffffffff83277293 in do_initcall_level (command_line=0xffff888003e2f900 "root", level=0x6) at ../init/main.c:1319
> >    #19 do_initcalls () at ../init/main.c:1335
> >    #20 do_basic_setup () at ../init/main.c:1354
> >    #21 kernel_init_freeable () at ../init/main.c:1571
> >    #22 0xffffffff81f64be1 in kernel_init (unused=<optimized out>) at ../init/main.c:1462
> >    #23 0xffffffff81001f49 in ret_from_fork () at ../arch/x86/entry/entry_64.S:308
> >    #24 0x0000000000000000 in ?? ()
> > 
> > Fix the problem by preventing xen_grant_init_backend_domid() from
> > setting dom0 as a backend when running in dom0.
> > 
> > Fixes: 035e3a4321f7 ("xen/virtio: Optimize the setup of "xen-grant-dma" devices")
> 
> 
> I am not 100% sure whether the Fixes tag points to precise commit. If I 
> am not mistaken, the said commit just moves the code in the context 
> without changing the logic of CONFIG_XEN_VIRTIO_FORCE_GRANT, this was 
> introduced before.
> 
> 
> > Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
> > ---
> >   drivers/xen/grant-dma-ops.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> > index 76f6f26265a3..29ed27ac450e 100644
> > --- a/drivers/xen/grant-dma-ops.c
> > +++ b/drivers/xen/grant-dma-ops.c
> > @@ -362,7 +362,9 @@ static int xen_grant_init_backend_domid(struct device *dev,
> >   	if (np) {
> >   		ret = xen_dt_grant_init_backend_domid(dev, np, backend_domid);
> >   		of_node_put(np);
> > -	} else if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_pv_domain()) {
> > +	} else if ((IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) ||
> > +		    xen_pv_domain()) &&
> > +		   !xen_initial_domain()) {
> 
> The commit lgtm, just one note:
> 
> 
> I would even bail out early in xen_virtio_restricted_mem_acc() instead,
> as I assume the same issue could happen on Arm with DT (although there 
> we don't guess the backend's domid, we read it from DT and quite 
> unlikely we get Dom0 being in Dom0 with correct DT).
> 
> Something like:
> 
> @@ -416,6 +421,10 @@ bool xen_virtio_restricted_mem_acc(struct 
> virtio_device *dev)
>   {
>          domid_t backend_domid;
> 
> +       /* Xen grant DMA ops are not used when running as initial domain */
> +       if (xen_initial_domain())
> +               return false;
> +
>          if (!xen_grant_init_backend_domid(dev->dev.parent, 
> &backend_domid)) {
>                  xen_grant_setup_dma_ops(dev->dev.parent, backend_domid);
>                  return true;
> (END)
> 
> 
> 
> If so, that commit subject would need to be updated accordingly.
> 
> Let's see what other reviewers will say.

This doesn't work in all cases. Imagine using PCI Passthrough to assign
a "physical" virtio device to a domU. The domU will run into the same
error, right?

The problem is that we need a way for the virtio backend to advertise
its ability of handling grants. Right now we only have a way to do with
that with device tree on ARM. On x86, we only have
CONFIG_XEN_VIRTIO_FORCE_GRANT, and if we take
CONFIG_XEN_VIRTIO_FORCE_GRANT at face value, it also enables grants for
"physical" virtio devices. Note that in this case we are fixing a
nested-virtualization bug, but there are actually physical
virtio-compatible devices out there. CONFIG_XEN_VIRTIO_FORCE_GRANT will
break those too.

I think we need to add a second way? It could be anything that can help
us distinguish between a non-grants-capable virtio backend and a
grants-capable virtio backend, such as:
- a string on xenstore
- a xen param
- a special PCI configuration register value
- something in the ACPI tables
- the QEMU machine type

Or at least should we change CONFIG_XEN_VIRTIO_FORCE_GRANT into a
command line parameter so that it can be disabled in cases like this
one?

I realize that fixing this problem properly takes a lot longer than
adding a trivial if (dom0) return; check in the code. If you cannot find
a good way to solve the problem or you don't have time to do that now
and you need this bug fixed quickly, then I would be OK with the if
(dom0) return; check but please add a detailed TODO in-code comment to
explain that this is just a hack and we are still looking for a real
solution.

The check itself I prefer the original position because I want to retain
the ability of using virtio frontends with grant on ARM in Dom0 (DomD
case).

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