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Date: Tue, 4 Jul 2023 08:25:21 +0200
From: Juergen Gross <jgross@...e.com>
To: Stefano Stabellini <sstabellini@...nel.org>,
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>,
vikram.garhwal@....com
Subject: Re: [PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0
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@...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.
>>
>>
>> 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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