[Qemu-devel] [PATCH v2 05/13] block: Switch to 64-bit bl.max_transfer

[Eric Blake]

This change has no semantic impact: all drivers either leave the
value at 0 (no inherent 32-bit limit is still translated into
fragmentation below 2G; see the previous patch for that audit), or
set it to a value less than 2G.  However, switching to a larger
type and enforcing the 2G cap at the block layer makes it easier
to audit specific drivers for their robustness to larger sizing,
by letting them specify a value larger than INT_MAX if they have
been audited to be 64-bit clean.

Signed-off-by: Eric Blake <address@hidden>
---
 include/block/block_int.h | 8 +++++---
 block/io.c                | 7 +++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index f605622216d..b19d94dac5f 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -549,9 +549,11 @@ typedef struct BlockLimits {

     /* Maximal transfer length in bytes.  Need not be power of 2, but
      * must be multiple of opt_transfer and bl.request_alignment, or 0
-     * for no 32-bit limit.  For now, anything larger than INT_MAX is
-     * clamped down. */
-    uint32_t max_transfer;
+     * for no 32-bit limit.  The block layer fragments all actions
+     * below 2G, so setting this value to anything larger than INT_MAX
+     * implies that the driver has been audited for 64-bit
+     * cleanness. */
+    uint64_t max_transfer;

     /* memory alignment, in bytes so that no bounce buffer is needed */
     size_t min_mem_alignment;
diff --git a/block/io.c b/block/io.c
index 39d4e7a3ae1..4911a1123eb 100644
--- a/block/io.c
+++ b/block/io.c
@@ -159,6 +159,13 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error 
**errp)
     if (drv->bdrv_refresh_limits) {
         drv->bdrv_refresh_limits(bs, errp);
     }
+
+    /* Clamp max_transfer to 2G */
+    if (bs->bl.max_transfer > UINT32_MAX) {
+        bs->bl.max_transfer = QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
+                                              MAX(bs->bl.opt_transfer,
+                                                  bs->bl.request_alignment));
+    }
 }

 /**
-- 
2.17.2