Re: [PATCH 1/5] Current QEMU and Linux drivers can export queue parameters via the virtio-blk configuration space. Document this, since the next patch will have to add another configuration field after these.

[Rusty Russell <rusty@au1.ibm.com>]

(Re-xmit: sent from wrong email addr last time)

Paolo Bonzini <pbonzini@redhat.com> writes:
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Thanks!

I turned it into this diff against current SVN:

commit 6045904929c3154884fa5d77d9a1748e096d333d
Author: Rusty Russell <rusty@au1.ibm.com>
Date:   Tue Aug 20 14:10:57 2013 +0930

    2.5.2.3: Add block topology.
    
    From: Paolo Bonzini <pbonzini@redhat.com>
    
    Current QEMU and Linux drivers can export queue parameters via the
    virtio-blk configuration space.  Document this, since the next patch
    will have to add another configuration field after these.
    
    See: https://lists.oasis-open.org/archives/virtio-comment/201308/msg00028.html
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 517eb07..d8302cb 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1538,6 +1538,9 @@ device except where noted.
 
   VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
 
+  VIRTIO_BLK_F_TOPOLOGY (10) Device exports information on optimal I/O
+    alignment.
+
   Device configuration layout The capacity of the device 
   (expressed in 512-byte sectors) is always present. The 
   availability of the others all depend on various feature bits 
@@ -1553,6 +1556,12 @@ device except where noted.
 			u8 sectors;
 		} geometry;
 		le32 blk_size;
+		struct virtio_blk_topology {
+			u8 physical_block_exp;
+			u8 alignment_offset;
+			u16 min_io_size;
+			u32 opt_io_size;
+		} topology;
 	};
 
 2.5.2.4 Device Initialization
@@ -1564,13 +1573,18 @@ device except where noted.
 
 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the 
   blk_size field can be read to determine the optimal sector size 
-  for the driver to use. This does not effect the units used in 
+  for the driver to use. This does not affect the units used in 
   the protocol (always 512 bytes), but awareness of the correct 
-  value can effect performance.
+  value can affect performance.
 
 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write 
   requests will fail.
 
+4. If the VIRTIO_BLK_F_TOPOLOGY feature is negotiated, the fields in the
+  topology struct can be read to determine the physical block size and optimal
+  I/O lengths for the driver to use. This also does not affect the units
+  in the protocol, only performance.
+
 2.5.2.5 Device Operation
 ------------------------