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

[Juergen Gross <jgross@suse.com>]

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On 30.06.23 00:44, Stefano Stabellini wrote:
> On Thu, 29 Jun 2023, Oleksandr Tyshchenko wrote:
>> 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@suse.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.
>>
>>
>> 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.
> 
> Yes, I would imagine virtio_dev_probe() would be called for them too
> 
> 
> 
>> 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.
> 
> xenstore would be among the easiest to make work. The only downside is
> the dependency on xenstore which otherwise virtio+grants doesn't have.

I'm in favor of using xenstore. Especially the hotplug support would be
much easier using xenstore (the alternative would be ACPI, which I don't
think we want for PV guests or many Arm configurations).


Juergen

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