Re: [PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0

From: Oleksandr Tyshchenko
Date: Thu Jun 29 2023 - 16:30:11 EST

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On 29.06.23 04:00, Stefano Stabellini wrote:

Hello Stefano

> 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@entry=0xffff8880074a8400, len=len@entry=0xffffc90000413c94, ctx=ctx@entry=0x0 <fixed_percpu_data>) at ../drivers/virtio/virtio_ring.c:2326
>>> #1 0xffffffff817086b7 in virtqueue_get_buf (_vq=_vq@entry=0xffff8880074a8400, len=len@entry=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@entry=0xffff88800585e810, drv=drv@entry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:658
>>> #8 0xffffffff8198e58f in __driver_probe_device (drv=drv@entry=0xffffffff82be40c0 <virtio_console>, dev=dev@entry=0xffff88800585e810) at ../drivers/base/dd.c:800
>>> #9 0xffffffff8198e65a in driver_probe_device (drv=drv@entry=0xffffffff82be40c0 <virtio_console>, dev=dev@entry=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@entry=0x0 <fixed_percpu_data>, data=data@entry=0xffffffff82be40c0 <virtio_console>,
>>> fn=fn@entry=0xffffffff8198e7b0 <__driver_attach>) at ../drivers/base/bus.c:368
>>> #12 0xffffffff8198db65 in driver_attach (drv=drv@entry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/dd.c:1233
>>> #13 0xffffffff8198d207 in bus_add_driver (drv=drv@entry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/bus.c:673
>>> #14 0xffffffff8198f550 in driver_register (drv=drv@entry=0xffffffff82be40c0 <virtio_console>) at ../drivers/base/driver.c:246
>>> #15 0xffffffff81706b47 in register_virtio_driver (driver=driver@entry=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@xxxxxxxx>
>>> ---
>>> 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.

If these "physical" virtio devices are also spawned by
drivers/virtio/virtio.c:virtio_dev_probe(), then yes, otherwise I don't
see how this could even be possible, but I might miss something here.

xen_virtio_restricted_mem_acc() gets called indirectly from
virtio_dev_probe()->virtio_features_ok()->
virtio_check_mem_acc_cb(). So the Xen grant DMA ops are only installed
for those.

>
> 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

Yes, I remember there was a discussion regarding that. The point is to
choose a solution to be functional for both PV and HVM *and* to be able
to support a hotplug. IIRC, the xenstore could be a possible candidate.

>
> 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?

IIUC, this will help with HVM only.

>
> 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).

Makes sense, agree.

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