Re: [PATCH v2 7/8] xen_disk: use a single entry iovec

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Paul Durrant [mailto:paul.durrant@citrix.com]
> Sent: 04 May 2018 14:56
> To: xen-devel@lists.xenproject.org; qemu-block@nongnu.org; qemu-
> devel@nongnu.org
> Cc: Paul Durrant <Paul.Durrant@citrix.com>; Stefano Stabellini
> <sstabellini@kernel.org>; Anthony Perard <anthony.perard@citrix.com>;
> Kevin Wolf <kwolf@redhat.com>; Max Reitz <mreitz@redhat.com>
> Subject: [PATCH v2 7/8] xen_disk: use a single entry iovec
> 
> Since xen_disk now always copies data to and from a guest there is no need
> to maintain a vector entry corresponding to every page of a request.
> This means there is less per-request state to maintain so the ioreq
> structure can shrink significantly.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Max Reitz <mreitz@redhat.com>
> 
> v2:
>  - Re-based

Unfortunately I managed to drop a hunk during rebase and so this patch is actually broken. I'll send a rectified v3 shortly.

  Paul

> ---
>  hw/block/xen_disk.c | 71 ++++++++++++++---------------------------------------
>  1 file changed, 18 insertions(+), 53 deletions(-)
> 
> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 28be8b6..230961f 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -46,13 +46,10 @@ struct ioreq {
>      /* parsed request */
>      off_t               start;
>      QEMUIOVector        v;
> +    void                *buf;
> +    size_t              size;
>      int                 presync;
> 
> -    /* grant mapping */
> -    uint32_t            refs[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> -    void                *page[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> -    void                *pages;
> -
>      /* aio status */
>      int                 aio_inflight;
>      int                 aio_errors;
> @@ -110,12 +107,10 @@ static void ioreq_reset(struct ioreq *ioreq)
>      memset(&ioreq->req, 0, sizeof(ioreq->req));
>      ioreq->status = 0;
>      ioreq->start = 0;
> +    ioreq->buf = NULL;
> +    ioreq->size = 0;
>      ioreq->presync = 0;
> 
> -    memset(ioreq->refs, 0, sizeof(ioreq->refs));
> -    memset(ioreq->page, 0, sizeof(ioreq->page));
> -    ioreq->pages = NULL;
> -
>      ioreq->aio_inflight = 0;
>      ioreq->aio_errors = 0;
> 
> @@ -138,7 +133,7 @@ static struct ioreq *ioreq_start(struct XenBlkDev
> *blkdev)
>          ioreq = g_malloc0(sizeof(*ioreq));
>          ioreq->blkdev = blkdev;
>          blkdev->requests_total++;
> -        qemu_iovec_init(&ioreq->v, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +        qemu_iovec_init(&ioreq->v, 1);
>      } else {
>          /* get one from freelist */
>          ioreq = QLIST_FIRST(&blkdev->freelist);
> @@ -183,7 +178,6 @@ static void ioreq_release(struct ioreq *ioreq, bool
> finish)
>  static int ioreq_parse(struct ioreq *ioreq)
>  {
>      struct XenBlkDev *blkdev = ioreq->blkdev;
> -    uintptr_t mem;
>      size_t len;
>      int i;
> 
> @@ -230,13 +224,10 @@ static int ioreq_parse(struct ioreq *ioreq)
>              goto err;
>          }
> 
> -        ioreq->refs[i]   = ioreq->req.seg[i].gref;
> -
> -        mem = ioreq->req.seg[i].first_sect * blkdev->file_blk;
>          len = (ioreq->req.seg[i].last_sect - ioreq->req.seg[i].first_sect + 1) *
> blkdev->file_blk;
> -        qemu_iovec_add(&ioreq->v, (void*)mem, len);
> +        ioreq->size += len;
>      }
> -    if (ioreq->start + ioreq->v.size > blkdev->file_size) {
> +    if (ioreq->start + ioreq->size > blkdev->file_size) {
>          xen_pv_printf(&blkdev->xendev, 0, "error: access beyond end of
> file\n");
>          goto err;
>      }
> @@ -247,35 +238,6 @@ err:
>      return -1;
>  }
> 
> -static void ioreq_free_copy_buffers(struct ioreq *ioreq)
> -{
> -    int i;
> -
> -    for (i = 0; i < ioreq->v.niov; i++) {
> -        ioreq->page[i] = NULL;
> -    }
> -
> -    qemu_vfree(ioreq->pages);
> -}
> -
> -static int ioreq_init_copy_buffers(struct ioreq *ioreq)
> -{
> -    int i;
> -
> -    if (ioreq->v.niov == 0) {
> -        return 0;
> -    }
> -
> -    ioreq->pages = qemu_memalign(XC_PAGE_SIZE, ioreq->v.niov *
> XC_PAGE_SIZE);
> -
> -    for (i = 0; i < ioreq->v.niov; i++) {
> -        ioreq->page[i] = ioreq->pages + i * XC_PAGE_SIZE;
> -        ioreq->v.iov[i].iov_base = ioreq->page[i];
> -    }
> -
> -    return 0;
> -}
> -
>  static int ioreq_grant_copy(struct ioreq *ioreq)
>  {
>      struct XenBlkDev *blkdev = ioreq->blkdev;
> @@ -284,6 +246,7 @@ static int ioreq_grant_copy(struct ioreq *ioreq)
>      int i, count, rc;
>      int64_t file_blk = ioreq->blkdev->file_blk;
>      bool to_domain = (ioreq->req.operation == BLKIF_OP_READ);
> +    void *virt = ioreq->buf;
> 
>      if (ioreq->v.niov == 0) {
>          return 0;
> @@ -293,16 +256,17 @@ static int ioreq_grant_copy(struct ioreq *ioreq)
> 
>      for (i = 0; i < count; i++) {
>          if (to_domain) {
> -            segs[i].dest.foreign.ref = ioreq->refs[i];
> +            segs[i].dest.foreign.ref = ioreq->req.seg[i].gref;
>              segs[i].dest.foreign.offset = ioreq->req.seg[i].first_sect * file_blk;
> -            segs[i].source.virt = ioreq->v.iov[i].iov_base;
> +            segs[i].source.virt = virt;
>          } else {
> -            segs[i].source.foreign.ref = ioreq->refs[i];
> +            segs[i].source.foreign.ref = ioreq->req.seg[i].gref;
>              segs[i].source.foreign.offset = ioreq->req.seg[i].first_sect * file_blk;
> -            segs[i].dest.virt = ioreq->v.iov[i].iov_base;
> +            segs[i].dest.virt = virt;
>          }
>          segs[i].len = (ioreq->req.seg[i].last_sect
>                         - ioreq->req.seg[i].first_sect + 1) * file_blk;
> +        virt += segs[i].len;
>      }
> 
>      rc = xen_be_copy_grant_refs(xendev, to_domain, segs, count);
> @@ -314,6 +278,7 @@ static int ioreq_grant_copy(struct ioreq *ioreq)
>          return -1;
>      }
> 
> +    qemu_iovec_add(&ioreq->v, ioreq->buf, ioreq->size);
>      return rc;
>  }
> 
> @@ -348,14 +313,14 @@ static void qemu_aio_complete(void *opaque, int
> ret)
>          if (ret == 0) {
>              ioreq_grant_copy(ioreq);
>          }
> -        ioreq_free_copy_buffers(ioreq);
> +        qemu_vfree(ioreq->buf);
>          break;
>      case BLKIF_OP_WRITE:
>      case BLKIF_OP_FLUSH_DISKCACHE:
>          if (!ioreq->req.nr_segments) {
>              break;
>          }
> -        ioreq_free_copy_buffers(ioreq);
> +        qemu_vfree(ioreq->buf);
>          break;
>      default:
>          break;
> @@ -423,12 +388,12 @@ static int ioreq_runio_qemu_aio(struct ioreq
> *ioreq)
>  {
>      struct XenBlkDev *blkdev = ioreq->blkdev;
> 
> -    ioreq_init_copy_buffers(ioreq);
> +    ioreq->buf = qemu_memalign(XC_PAGE_SIZE, ioreq->size);
>      if (ioreq->req.nr_segments &&
>          (ioreq->req.operation == BLKIF_OP_WRITE ||
>           ioreq->req.operation == BLKIF_OP_FLUSH_DISKCACHE) &&
>          ioreq_grant_copy(ioreq)) {
> -        ioreq_free_copy_buffers(ioreq);
> +        qemu_vfree(ioreq->buf);
>          goto err;
>      }
> 
> --
> 2.1.4


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