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

Feng Liu via Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx> · Wed, 8 Mar 2023 11:44:35 -0500

On 2023-03-08 a.m.11:25, Michael S. Tsirkin wrote:
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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:
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On Tue, Mar 07, 2023 at 09:17:55PM +0000, Feng Liu wrote:
On 2023-03-07 04:14, David Edmondson wrote:
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Feng Liu via Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>
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.


Hi, Michael
Ok, I see. Thanks for your explaination. I will push the next version of 
this patch after you post




Signed-off-by: Feng Liu <feliu@xxxxxxxxxx>
Reviewed-by: Jiri Pirko <jiri@xxxxxxxxxx>
Reviewed-by: Parav Pandit <parav@xxxxxxxxxx>
Reviewed-by: Gavin Li <gavinl@xxxxxxxxxx>
Reviewed-by: Bodong Wang <bodong@xxxxxxxxxx>
---
    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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