linux-kernel - Re: [RFC PATCH 10/10] vhost/vdpa: return configuration bytes read and written to user space

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Date:   Mon, 8 Mar 2021 11:59:12 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH 10/10] vhost/vdpa: return configuration bytes read and
 written to user space


On 2021/3/5 4:37 下午, Stefano Garzarella wrote:
> On Thu, Mar 04, 2021 at 04:31:22PM +0800, Jason Wang wrote:
>>
>> On 2021/3/2 10:06 下午, Stefano Garzarella wrote:
>>> On Tue, Mar 02, 2021 at 12:05:35PM +0800, Jason Wang wrote:
>>>>
>>>> On 2021/2/16 5:44 下午, Stefano Garzarella wrote:
>>>>> vdpa_get_config() and vdpa_set_config() now return the amount
>>>>> of bytes read and written, so let's return them to the user space.
>>>>>
>>>>> We also modify vhost_vdpa_config_validate() to return 0 (bytes read
>>>>> or written) instead of an error, when the buffer length is 0.
>>>>>
>>>>> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
>>>>> ---
>>>>>  drivers/vhost/vdpa.c | 26 +++++++++++++++-----------
>>>>>  1 file changed, 15 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>>>>> index 21eea2be5afa..b754c53171a7 100644
>>>>> --- a/drivers/vhost/vdpa.c
>>>>> +++ b/drivers/vhost/vdpa.c
>>>>> @@ -191,9 +191,6 @@ static ssize_t 
>>>>> vhost_vdpa_config_validate(struct vhost_vdpa *v,
>>>>>      struct vdpa_device *vdpa = v->vdpa;
>>>>>      u32 size = vdpa->config->get_config_size(vdpa);
>>>>> -    if (c->len == 0)
>>>>> -        return -EINVAL;
>>>>> -
>>>>>      return min(c->len, size);
>>>>>  }
>>>>> @@ -204,6 +201,7 @@ static long vhost_vdpa_get_config(struct 
>>>>> vhost_vdpa *v,
>>>>>      struct vhost_vdpa_config config;
>>>>>      unsigned long size = offsetof(struct vhost_vdpa_config, buf);
>>>>>      ssize_t config_size;
>>>>> +    long ret;
>>>>>      u8 *buf;
>>>>>      if (copy_from_user(&config, c, size))
>>>>> @@ -217,15 +215,18 @@ static long vhost_vdpa_get_config(struct 
>>>>> vhost_vdpa *v,
>>>>>      if (!buf)
>>>>>          return -ENOMEM;
>>>>> -    vdpa_get_config(vdpa, config.off, buf, config_size);
>>>>> -
>>>>> -    if (copy_to_user(c->buf, buf, config_size)) {
>>>>> -        kvfree(buf);
>>>>> -        return -EFAULT;
>>>>> +    ret = vdpa_get_config(vdpa, config.off, buf, config_size);
>>>>> +    if (ret < 0) {
>>>>> +        ret = -EFAULT;
>>>>> +        goto out;
>>>>>      }
>>>>> +    if (copy_to_user(c->buf, buf, config_size))
>>>>> +        ret = -EFAULT;
>>>>> +
>>>>> +out:
>>>>>      kvfree(buf);
>>>>> -    return 0;
>>>>> +    return ret;
>>>>>  }
>>>>>  static long vhost_vdpa_set_config(struct vhost_vdpa *v,
>>>>> @@ -235,6 +236,7 @@ static long vhost_vdpa_set_config(struct 
>>>>> vhost_vdpa *v,
>>>>>      struct vhost_vdpa_config config;
>>>>>      unsigned long size = offsetof(struct vhost_vdpa_config, buf);
>>>>>      ssize_t config_size;
>>>>> +    long ret;
>>>>>      u8 *buf;
>>>>>      if (copy_from_user(&config, c, size))
>>>>> @@ -248,10 +250,12 @@ static long vhost_vdpa_set_config(struct 
>>>>> vhost_vdpa *v,
>>>>>      if (IS_ERR(buf))
>>>>>          return PTR_ERR(buf);
>>>>> -    vdpa_set_config(vdpa, config.off, buf, config_size);
>>>>> +    ret = vdpa_set_config(vdpa, config.off, buf, config_size);
>>>>> +    if (ret < 0)
>>>>> +        ret = -EFAULT;
>>>>>      kvfree(buf);
>>>>> -    return 0;
>>>>> +    return ret;
>>>>>  }
>>>>
>>>>
>>>> So I wonder whether it's worth to return the number of bytes since 
>>>> we can't propogate the result to driver or driver doesn't care 
>>>> about that.
>>>
>>> Okay, but IIUC user space application that issue 
>>> VHOST_VDPA_GET_CONFIG ioctl can use the return value.
>>
>>
>> Yes, but it looks to it's too late to change since it's a userspace 
>> noticble behaviour.
>
> Yeah, this is a good point.
> I looked at QEMU and we only check if the value is not negative, so it 
> should work, but for other applications it could be a real change.
>
> Do we leave it as is?


Yes, I think we'd better be conservative here.

Thanks


>
>>
>>
>>>
>>> Should we change also 'struct virtio_config_ops' to propagate this 
>>> value also to virtio drivers?
>>
>>
>> I think not, the reason is the driver doesn't expect the get()/set() 
>> can fail...
>
> Got it.
>
> Thanks,
> Stefano
>