[Qemu-devel] [PATCH v2 17/18] block: default min_io_size to host block size when doing rmw

[Paolo Bonzini <pbonzini@redhat.com>]

Same as the previous patch.  However, since the min_io_size is a byte
value rather than an exponent, we want to make sure that we pass a 0
when running on 512b-sector disks.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block.c         |   17 +++++++++++++++++
 block.h         |    1 +
 hw/scsi-disk.c  |    2 +-
 hw/virtio-blk.c |    2 +-
 4 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 0fce655..4880143 100644
--- a/block.c
+++ b/block.c
@@ -4153,3 +4153,20 @@ unsigned int get_physical_block_exp(BlockConf *conf)
 
     return exp;
 }
+
+unsigned int get_min_io_size(BlockConf *conf)
+{
+    if (conf->min_io_size || !conf->bs) {
+        return conf->min_io_size;
+    }
+
+    /* Ask the OS to avoid read-modify-write cycles.  But when running
+     * on 512b-sector disks, we should not modify the parameters that
+     * guests had seen so far.
+     */
+    if (conf->bs->host_block_size > conf->logical_block_size) {
+        return conf->bs->host_block_size;
+    }
+
+    return 0;
+}
diff --git a/block.h b/block.h
index 4270f67..ad1d18c 100644
--- a/block.h
+++ b/block.h
@@ -406,6 +406,7 @@ typedef struct BlockConf {
 } BlockConf;
 
 unsigned int get_physical_block_exp(BlockConf *conf);
+unsigned int get_min_io_size(BlockConf *conf);
 
 #define DEFINE_BLOCK_PROPERTIES(_state, _conf)                          \
     DEFINE_PROP_DRIVE("drive", _state, _conf.bs),                       \
diff --git a/hw/scsi-disk.c b/hw/scsi-disk.c
index 4f370a6..032ccf1 100644
--- a/hw/scsi-disk.c
+++ b/hw/scsi-disk.c
@@ -429,7 +429,7 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
             unsigned int unmap_sectors =
                     s->qdev.conf.discard_granularity / s->qdev.blocksize;
             unsigned int min_io_size =
-                    s->qdev.conf.min_io_size / s->qdev.blocksize;
+                    get_min_io_size(&s->qdev.conf) / s->qdev.blocksize;
             unsigned int opt_io_size =
                     s->qdev.conf.opt_io_size / s->qdev.blocksize;
 
diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 7c44a1f..06b5c0d 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -491,7 +491,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
     stl_raw(&blkcfg.seg_max, 128 - 2);
     stw_raw(&blkcfg.cylinders, cylinders);
     stl_raw(&blkcfg.blk_size, blk_size);
-    stw_raw(&blkcfg.min_io_size, s->conf->min_io_size / blk_size);
+    stw_raw(&blkcfg.min_io_size, get_min_io_size(s->conf) / blk_size);
     stw_raw(&blkcfg.opt_io_size, s->conf->opt_io_size / blk_size);
     blkcfg.heads = heads;
     blkcfg.sectors = secs & ~s->sector_mask;
-- 
1.7.7.6