Re: [v4] virtio-gpu: use consistent error checking style for virtio_gpu_create_mapping_iov

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 11/18/25 15:32, Honglei Huang wrote:
> 
> 
> On 2025/11/18 09:48, Dmitry Osipenko wrote:
>> On 11/17/25 16:22, Markus Armbruster wrote:
>>> Dmitry Osipenko <dmitry.osipenko@collabora.com> writes:
>>>
>>>> On 11/17/25 13:51, Honglei Huang wrote:
>>>>> diff --git a/hw/display/virtio-gpu-rutabaga.c b/hw/display/virtio-
>>>>> gpu-rutabaga.c
>>>>> index ed5ae52acb..ea2928b706 100644
>>>>> --- a/hw/display/virtio-gpu-rutabaga.c
>>>>> +++ b/hw/display/virtio-gpu-rutabaga.c
>>>>> @@ -466,7 +466,7 @@ rutabaga_cmd_attach_backing(VirtIOGPU *g,
>>>>> struct virtio_gpu_ctrl_command *cmd)
>>>>>         ret = virtio_gpu_create_mapping_iov(g, att_rb.nr_entries,
>>>>> sizeof(att_rb),
>>>>>                                           cmd, NULL, &res->iov,
>>>>> &res->iov_cnt);
>>>>> -    CHECK(!ret, cmd);
>>>>> +    CHECK(ret >= 0, cmd);
>>>>
>>>> virtio_gpu_create_mapping_iov() doesn't return positive values, don't
>>>> see how this change improves anything. You now saying that ret > 0 is
>>>> okay, while it shall never happen.
>>>
>>> Please see
>>>
>>>      Subject: Re: [PATCH] virtio-gpu-virgl: fix error handling in
>>> virgl_cmd_resource_create_blob
>>>      Date: Mon, 17 Nov 2025 08:49:42 +0100
>>>      Message-ID: <87ms4lrtd5.fsf@pond.sub.org>
>>>      https://lore.kernel.org/qemu-devel/87ms4lrtd5.fsf@pond.sub.org/
>>
>> It's a rather common bug when errno isn't negated by mistake and a
>> positive error code is returned. Ignoring positive values when they
>> aren't expected opens door to unnecessary problems, IMO.
>>
> 
> How about apply the v2 or v3 firstly to fix the
> virtio_gpu_create_mapping_iov() block issue in virtio-gpu?
> 
> I will create another thread for the `CHECK(!ret, cmd);` thing in rutabaga.

There was a precedent of virtio-gpu not handling positive error codes
properly [1]. To me there is no problem that needs to be fixed when
virtio_gpu_create_mapping_iov() is never expected to return positive
values and doesn't return them.

[1]
https://lore.kernel.org/qemu-devel/20240129073921.446869-1-dmitry.osipenko@collabora.com/

It's a common expectation that errors are negative. But in practice it's
not always true, especially when interacting with external code.

Functionally this patch doesn't change anything. Will leave to Alex and
Akihiko to decide on it.

-- 
Best regards,
Dmitry