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

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Date:   Fri, 5 Mar 2021 09:37:12 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jason Wang <jasowang@...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 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?

>
>
>>
>>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