Re: [PATCH 3/3] virtio_ring: Use const to annotate read-only pointer params

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 08, 2023 at 10:59:57AM -0500, Feng Liu wrote:
> 
> 
> On 2023-03-08 a.m.9:13, Michael S. Tsirkin wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Tue, Mar 07, 2023 at 09:17:55PM +0000, Feng Liu wrote:
> > > On 2023-03-07 04:14, David Edmondson wrote:
> > > > External email: Use caution opening links or attachments
> > > > 
> > > > 
> > > > Feng Liu via Virtualization <virtualization@lists.linux-foundation.org>
> > > > writes:
> > > > 
> > > > > Add const to make the read-only pointer parameters clear, similar to
> > > > > many existing functions.
> > > > 
> > > > In many of the modified functions the local variable that is a cast of
> > > > the argument could also be const. Is there a reason not to do both at
> > > > the same time?
> > > > 
> > > 
> > > Hi,David
> > > 
> > > In order to prevent the content of a pointer parameter from being
> > > modified and increase the readability of the function, it is recommended
> > > to add the 'const' keyword to the parameter. This is not necessary for
> > > local variables and non-pointer parameters, as they are only stored on
> > > the stack and do not affect the original value or structure member
> > > passed into the function. Therefore, in this case, the 'const' keyword
> > > is only added to pointer parameters.
> > 
> > This makes no sense to me. If ytou cast away the const then it is
> > pointless.
> > 
> 
> Hi, Michael
> 
> I really don't quite understand your point of view.
> Is a local variable that needs to be add const? Can you help to point
> out the specific problem/point ?

I just repeated what David said.  Basically most of these functions use
to_vvq which uses container_of which in turn loses const qualifier.
So your change is poinless since rest of code accesses vq through
to_vvq.

What to do? I don't like the idea of to_vvq_const.
So I propose a version of container_of using _Generic
which preserves the const qualifier.


#define container_of(ptr, type, member) \
    ({ \
        const void *__mptr = (ptr); \
        static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
                      __same_type(*(ptr), void),                        \
                      "pointer type mismatch in container_of()");       \
        _Generic((ptr), \
                 typeof(&((const type *)0)->member): \
                 (const type *)(__mptr - offsetof(type, member)), \
                 default: \
                 (type *)(__mptr - offsetof(type, member))); \
    })


I'll hack it up in a day or two and post.




> > > 
> > > > > Signed-off-by: Feng Liu <feliu@nvidia.com>
> > > > > Reviewed-by: Jiri Pirko <jiri@nvidia.com>
> > > > > Reviewed-by: Parav Pandit <parav@nvidia.com>
> > > > > Reviewed-by: Gavin Li <gavinl@nvidia.com>
> > > > > Reviewed-by: Bodong Wang <bodong@nvidia.com>
> > > > > ---
> > > > >    drivers/virtio/virtio_ring.c | 25 ++++++++++++-------------
> > > > >    include/linux/virtio.h       | 12 ++++++------
> > > > >    2 files changed, 18 insertions(+), 19 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 806cc44eae64..8010fd1d2047 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -233,7 +233,7 @@ static void vring_free(struct virtqueue *_vq);
> > > > > 
> > > > >    #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
> > > > > 
> > > > > -static bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> > > > > +static bool virtqueue_use_indirect(const struct vring_virtqueue *vq,
> > > > >                                           unsigned int total_sg)
> > > > >    {
> > > > >         /*
> > > > > @@ -269,7 +269,7 @@ static bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> > > > >     * unconditionally on data path.
> > > > >     */
> > > > > 
> > > > > -static bool vring_use_dma_api(struct virtio_device *vdev)
> > > > > +static bool vring_use_dma_api(const struct virtio_device *vdev)
> > > > >    {
> > > > >         if (!virtio_has_dma_quirk(vdev))
> > > > >                 return true;
> > > > > @@ -289,7 +289,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> > > > >         return false;
> > > > >    }
> > > > > 
> > > > > -size_t virtio_max_dma_size(struct virtio_device *vdev)
> > > > > +size_t virtio_max_dma_size(const struct virtio_device *vdev)
> > > > >    {
> > > > >         size_t max_segment_size = SIZE_MAX;
> > > > > 
> > > > > @@ -423,7 +423,7 @@ static void virtqueue_init(struct vring_virtqueue *vq, u32 num)
> > > > >     */
> > > > > 
> > > > >    static void vring_unmap_one_split_indirect(const struct vring_virtqueue *vq,
> > > > > -                                        struct vring_desc *desc)
> > > > > +                                        const struct vring_desc *desc)
> > > > >    {
> > > > >         u16 flags;
> > > > > 
> > > > > @@ -1183,7 +1183,7 @@ static u16 packed_last_used(u16 last_used_idx)
> > > > >    }
> > > > > 
> > > > >    static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > > > > -                                  struct vring_desc_extra *extra)
> > > > > +                                  const struct vring_desc_extra *extra)
> > > > >    {
> > > > >         u16 flags;
> > > > > 
> > > > > @@ -1206,7 +1206,7 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > > > >    }
> > > > > 
> > > > >    static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> > > > > -                                struct vring_packed_desc *desc)
> > > > > +                                 const struct vring_packed_desc *desc)
> > > > >    {
> > > > >         u16 flags;
> > > > > 
> > > > > @@ -2786,7 +2786,7 @@ EXPORT_SYMBOL_GPL(vring_transport_features);
> > > > >     * Returns the size of the vring.  This is mainly used for boasting to
> > > > >     * userspace.  Unlike other operations, this need not be serialized.
> > > > >     */
> > > > > -unsigned int virtqueue_get_vring_size(struct virtqueue *_vq)
> > > > > +unsigned int virtqueue_get_vring_size(const struct virtqueue *_vq)
> > > > >    {
> > > > > 
> > > > >         struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > @@ -2819,7 +2819,7 @@ void __virtqueue_unbreak(struct virtqueue *_vq)
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(__virtqueue_unbreak);
> > > > > 
> > > > > -bool virtqueue_is_broken(struct virtqueue *_vq)
> > > > > +bool virtqueue_is_broken(const struct virtqueue *_vq)
> > > > >    {
> > > > >         struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > 
> > > > > @@ -2827,8 +2827,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(virtqueue_is_broken);
> > > > > 
> > > > > -/*
> > > > > - * This should prevent the device from being used, allowing drivers to
> > > > > +/ This should prevent the device from being used, allowing drivers to
> > > > >     * recover.  You may need to grab appropriate locks to flush.
> > > > >     */
> > > > >    void virtio_break_device(struct virtio_device *dev)
> > > > > @@ -2881,7 +2880,7 @@ dma_addr_t virtqueue_get_desc_addr(struct virtqueue *_vq)
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(virtqueue_get_desc_addr);
> > > > > 
> > > > > -dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> > > > > +dma_addr_t virtqueue_get_avail_addr(const struct virtqueue *_vq)
> > > > >    {
> > > > >         struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > 
> > > > > @@ -2895,7 +2894,7 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(virtqueue_get_avail_addr);
> > > > > 
> > > > > -dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> > > > > +dma_addr_t virtqueue_get_used_addr(const struct virtqueue *_vq)
> > > > >    {
> > > > >         struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > 
> > > > > @@ -2910,7 +2909,7 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> > > > >    EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
> > > > > 
> > > > >    /* Only available for split ring */
> > > > > -const struct vring *virtqueue_get_vring(struct virtqueue *vq)
> > > > > +const struct vring *virtqueue_get_vring(const struct virtqueue *vq)
> > > > >    {
> > > > >         return &to_vvq(vq)->split.vring;
> > > > >    }
> > > > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > > > index 2b472514c49b..36a79374e735 100644
> > > > > --- a/include/linux/virtio.h
> > > > > +++ b/include/linux/virtio.h
> > > > > @@ -84,14 +84,14 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> > > > > 
> > > > >    void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> > > > > 
> > > > > -unsigned int virtqueue_get_vring_size(struct virtqueue *vq);
> > > > > +unsigned int virtqueue_get_vring_size(const struct virtqueue *vq);
> > > > > 
> > > > > -bool virtqueue_is_broken(struct virtqueue *vq);
> > > > > +bool virtqueue_is_broken(const struct virtqueue *vq);
> > > > > 
> > > > > -const struct vring *virtqueue_get_vring(struct virtqueue *vq);
> > > > > -dma_addr_t virtqueue_get_desc_addr(struct virtqueue *vq);
> > > > > -dma_addr_t virtqueue_get_avail_addr(struct virtqueue *vq);
> > > > > -dma_addr_t virtqueue_get_used_addr(struct virtqueue *vq);
> > > > > +const struct vring *virtqueue_get_vring(const struct virtqueue *vq);
> > > > > +dma_addr_t virtqueue_get_desc_addr(const struct virtqueue *vq);
> > > > > +dma_addr_t virtqueue_get_avail_addr(const struct virtqueue *vq);
> > > > > +dma_addr_t virtqueue_get_used_addr(const struct virtqueue *vq);
> > > > > 
> > > > >    int virtqueue_resize(struct virtqueue *vq, u32 num,
> > > > >                      void (*recycle)(struct virtqueue *vq, void *buf));
> > > > > --
> > > > > 2.34.1
> > > > > 
> > > > > _______________________________________________
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> > > > > Virtualization@lists.linux-foundation.org
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> > > > --
> > > > Woke up in my clothes again this morning, don't know exactly where I am.
> > > 
> > 

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