[PATCH v4 2/7] scsi-generic: pass max_segments via max_iov field in BlockLimits

[Paolo Bonzini]

I/O to a disk via read/write is not limited by the number of segments allowed
by the host adapter; the kernel can split requests if needed, and the limit
imposed by the host adapter can be very low (256k or so) to avoid that SG_IO
returns EINVAL if memory is heavily fragmented.

Since this value is only interesting for SG_IO-based I/O, do not include
it in the max_transfer and only take it into account when patching the
block limits VPD page in the scsi-generic device.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/file-posix.c     | 3 +--
 hw/scsi/scsi-generic.c | 6 ++++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 536998a1d6..670c577bfe 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1239,8 +1239,7 @@ static void raw_refresh_limits(BlockDriverState *bs, 
Error **errp)
 
         ret = sg_get_max_segments(s->fd);
         if (ret > 0) {
-            bs->bl.max_transfer = MIN(bs->bl.max_transfer,
-                                      ret * qemu_real_host_page_size);
+            bs->bl.max_iov = ret;
         }
     }
 
diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
index 98c30c5d5c..82e1e2ee79 100644
--- a/hw/scsi/scsi-generic.c
+++ b/hw/scsi/scsi-generic.c
@@ -179,10 +179,12 @@ static void scsi_handle_inquiry_reply(SCSIGenericReq *r, 
SCSIDevice *s)
         (r->req.cmd.buf[1] & 0x01)) {
         page = r->req.cmd.buf[2];
         if (page == 0xb0) {
-            uint32_t max_transfer =
-                blk_get_max_transfer(s->conf.blk) / s->blocksize;
+            uint32_t max_transfer = blk_get_max_transfer(s->conf.blk);
+            uint32_t max_iov = blk_get_max_iov(s->conf.blk);
 
             assert(max_transfer);
+            max_transfer = MIN_NON_ZERO(max_transfer, max_iov * 
qemu_real_host_page_size)
+                / s->blocksize;
             stl_be_p(&r->buf[8], max_transfer);
             /* Also take care of the opt xfer len. */
             stl_be_p(&r->buf[12],
-- 
2.31.1