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[Qemu-devel] Re: [PATCH] Disable O_DIRECT for physical CDROM/DVD drives

From: Anthony Liguori
Subject: [Qemu-devel] Re: [PATCH] Disable O_DIRECT for physical CDROM/DVD drives
Date: Tue, 20 Jul 2010 10:40:05 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100528 Lightning/1.0b1 Thunderbird/3.0.5

On 07/20/2010 10:17 AM, address@hidden wrote:
From: Jes Sorensen<address@hidden>

O_DIRECT (cache=none) requires sector alignment, however the physical
sector size of CDROM/DVD drives is 2048, as opposed to most disk
devices which use 512. QEMU is hard coding 512 all over the place, so
allowing O_DIRECT for CDROM/DVD devices does not work.

Signed-off-by: Jes Sorensen<address@hidden>

Wouldn't a better solution be to have a cdrom_read/cdrom_write hook that did the appropriate bouncing?

Silently disabling something a user explicitly asked for is not a good option. In the very least, it should error out entirely.


Anthony Liguori

  block/raw-posix.c |    5 +++++
  1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 291699f..0ea79b6 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -1139,6 +1139,11 @@ static int cdrom_open(BlockDriverState *bs, const char 
*filename, int flags)
      BDRVRawState *s = bs->opaque;

      s->type = FTYPE_CD;
+    if (flags&  BDRV_O_NOCACHE) {
+        fprintf(stderr, "Disabling unsupported O_DIRECT (cache=none) for "
+                "CDROM/DVD device (%s)\n", filename);
+        flags&= ~BDRV_O_NOCACHE;
+    }

      /* open will not fail even if no CD is inserted, so add O_NONBLOCK */
      return raw_open_common(bs, filename, flags, O_NONBLOCK);

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