[Qemu-devel] [PATCH V2 2/6] virtio/console: Add a failback for unstealable pipe buffer

[Yoshihiro YUNOMAE]

From: Masami Hiramatsu <address@hidden>

Add a failback memcpy path for unstealable pipe buffer.
If buf->ops->steal() fails, virtio-serial tries to
copy the page contents to an allocated page, instead
of just failing splice().

Signed-off-by: Masami Hiramatsu <address@hidden>
---

 drivers/char/virtio_console.c |   28 +++++++++++++++++++++++++---
 1 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 730816c..22b7373 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -794,7 +794,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct 
pipe_buffer *buf,
                        struct splice_desc *sd)
 {
        struct sg_list *sgl = sd->u.data;
-       unsigned int len = 0;
+       unsigned int offset, len;
 
        if (sgl->n == MAX_SPLICE_PAGES)
                return 0;
@@ -807,9 +807,31 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct 
pipe_buffer *buf,
 
                len = min(buf->len, sd->len);
                sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
-               sgl->n++;
-               sgl->len += len;
+       } else {
+               /* Failback to copying a page */
+               struct page *page = alloc_page(GFP_KERNEL);
+               char *src = buf->ops->map(pipe, buf, 1);
+               char *dst;
+
+               if (!page)
+                       return -ENOMEM;
+               dst = kmap(page);
+
+               offset = sd->pos & ~PAGE_MASK;
+
+               len = sd->len;
+               if (len + offset > PAGE_SIZE)
+                       len = PAGE_SIZE - offset;
+
+               memcpy(dst + offset, src + buf->offset, len);
+
+               kunmap(page);
+               buf->ops->unmap(pipe, buf, src);
+
+               sg_set_page(&(sgl->sg[sgl->n]), page, len, offset);
        }
+       sgl->n++;
+       sgl->len += len;
 
        return len;
 }