[PATCH 0/2] scsi-disk: Add FUA write support

[PATCH 0/2] scsi-disk: Add FUA write support

[Alberto Faria]

Add support for Force Unit Access (FUA) writes. The first patch makes scsi-disk
devices advertise FUA support by default; FUA requests will be emulated through
a regular write followed by a flush. The second patch lets us avoid FUA
emulation when the underlying block driver supports it natively.

Alberto Faria (2):
  scsi-disk: Advertise FUA support by default
  scsi-disk: Add native FUA support

 hw/scsi/scsi-disk.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

-- 
2.48.1

[PATCH 1/2] scsi-disk: Advertise FUA support by default

[Alberto Faria]

FUA emulation code is already is place.

Signed-off-by: Alberto Faria <afaria@redhat.com>
---
 hw/scsi/scsi-disk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
index e7f738b484..8cf50845ab 100644
--- a/hw/scsi/scsi-disk.c
+++ b/hw/scsi/scsi-disk.c
@@ -3212,7 +3212,7 @@ static const Property scsi_hd_properties[] = {
     DEFINE_PROP_BIT("removable", SCSIDiskState, features,
                     SCSI_DISK_F_REMOVABLE, false),
     DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
-                    SCSI_DISK_F_DPOFUA, false),
+                    SCSI_DISK_F_DPOFUA, true),
     DEFINE_PROP_UINT64("wwn", SCSIDiskState, qdev.wwn, 0),
     DEFINE_PROP_UINT64("port_wwn", SCSIDiskState, qdev.port_wwn, 0),
     DEFINE_PROP_UINT16("port_index", SCSIDiskState, port_index, 0),
-- 
2.48.1

[PATCH 2/2] scsi-disk: Add native FUA support

[Alberto Faria]

Avoid emulating FUA when the driver supports it natively. This should
provide better performance than a full flush after the write.

Signed-off-by: Alberto Faria <afaria@redhat.com>
---
 hw/scsi/scsi-disk.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
index 8cf50845ab..ce48e20ee6 100644
--- a/hw/scsi/scsi-disk.c
+++ b/hw/scsi/scsi-disk.c
@@ -43,6 +43,7 @@
 #include "qemu/cutils.h"
 #include "trace.h"
 #include "qom/object.h"
+#include "block/block_int-common.h"
 
 #ifdef __linux
 #include <scsi/sg.h>
@@ -75,7 +76,7 @@ struct SCSIDiskClass {
      */
     DMAIOFunc       *dma_readv;
     DMAIOFunc       *dma_writev;
-    bool            (*need_fua_emulation)(SCSICommand *cmd);
+    bool            (*need_fua)(SCSICommand *cmd);
     void            (*update_sense)(SCSIRequest *r);
 };
 
@@ -86,6 +87,7 @@ typedef struct SCSIDiskReq {
     uint32_t sector_count;
     uint32_t buflen;
     bool started;
+    bool need_fua;
     bool need_fua_emulation;
     struct iovec iov;
     QEMUIOVector qiov;
@@ -553,7 +555,7 @@ static void scsi_read_data(SCSIRequest *req)
 
     first = !r->started;
     r->started = true;
-    if (first && r->need_fua_emulation) {
+    if (first && r->need_fua) {
         block_acct_start(blk_get_stats(s->qdev.conf.blk), &r->acct, 0,
                          BLOCK_ACCT_FLUSH);
         r->req.aiocb = blk_aio_flush(s->qdev.conf.blk, scsi_do_read_cb, r);
@@ -2384,7 +2386,9 @@ static int32_t scsi_disk_dma_command(SCSIRequest *req, uint8_t *buf)
         scsi_check_condition(r, SENSE_CODE(LBA_OUT_OF_RANGE));
         return 0;
     }
-    r->need_fua_emulation = sdc->need_fua_emulation(&r->req.cmd);
+    r->need_fua = sdc->need_fua(&r->req.cmd);
+    r->need_fua_emulation = r->need_fua &&
+        (blk_bs(s->qdev.conf.blk)->supported_write_flags & BDRV_REQ_FUA) == 0;
     if (r->sector_count == 0) {
         scsi_req_complete(&r->req, GOOD);
     }
@@ -3134,7 +3138,8 @@ BlockAIOCB *scsi_dma_writev(int64_t offset, QEMUIOVector *iov,
 {
     SCSIDiskReq *r = opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
-    return blk_aio_pwritev(s->qdev.conf.blk, offset, iov, 0, cb, cb_opaque);
+    int flags = r->need_fua && !r->need_fua_emulation ? BDRV_REQ_FUA : 0;
+    return blk_aio_pwritev(s->qdev.conf.blk, offset, iov, flags, cb, cb_opaque);
 }
 
 static char *scsi_property_get_loadparm(Object *obj, Error **errp)
@@ -3183,7 +3188,7 @@ static void scsi_disk_base_class_initfn(ObjectClass *klass, void *data)
     device_class_set_legacy_reset(dc, scsi_disk_reset);
     sdc->dma_readv = scsi_dma_readv;
     sdc->dma_writev = scsi_dma_writev;
-    sdc->need_fua_emulation = scsi_is_cmd_fua;
+    sdc->need_fua = scsi_is_cmd_fua;
 }
 
 static const TypeInfo scsi_disk_base_info = {
@@ -3335,7 +3340,7 @@ static void scsi_block_class_initfn(ObjectClass *klass, void *data)
     sdc->dma_readv   = scsi_block_dma_readv;
     sdc->dma_writev  = scsi_block_dma_writev;
     sdc->update_sense = scsi_block_update_sense;
-    sdc->need_fua_emulation = scsi_block_no_fua;
+    sdc->need_fua = scsi_block_no_fua;
     dc->desc = "SCSI block device passthrough";
     device_class_set_props(dc, scsi_block_properties);
     dc->vmsd  = &vmstate_scsi_disk_state;
-- 
2.48.1

Re: [PATCH 1/2] scsi-disk: Advertise FUA support by default

[Daniel P. Berrangé]

On Tue, Mar 04, 2025 at 03:52:31PM +0000, Alberto Faria wrote:
> FUA emulation code is already is place.
> 
> Signed-off-by: Alberto Faria <afaria@redhat.com>
> ---
>  hw/scsi/scsi-disk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> index e7f738b484..8cf50845ab 100644
> --- a/hw/scsi/scsi-disk.c
> +++ b/hw/scsi/scsi-disk.c
> @@ -3212,7 +3212,7 @@ static const Property scsi_hd_properties[] = {
>      DEFINE_PROP_BIT("removable", SCSIDiskState, features,
>                      SCSI_DISK_F_REMOVABLE, false),
>      DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
> -                    SCSI_DISK_F_DPOFUA, false),
> +                    SCSI_DISK_F_DPOFUA, true),

Should this come with machine type compat to prevent us advertizing FUA
to existing machine type versions ?

>      DEFINE_PROP_UINT64("wwn", SCSIDiskState, qdev.wwn, 0),
>      DEFINE_PROP_UINT64("port_wwn", SCSIDiskState, qdev.port_wwn, 0),
>      DEFINE_PROP_UINT16("port_index", SCSIDiskState, port_index, 0),

With regards,
Daniel
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Re: [PATCH 2/2] scsi-disk: Add native FUA support

[Kevin Wolf]

Am 04.03.2025 um 16:52 hat Alberto Faria geschrieben:
> Avoid emulating FUA when the driver supports it natively. This should
> provide better performance than a full flush after the write.
> 
> Signed-off-by: Alberto Faria <afaria@redhat.com>

Did you try out if you can see performance improvements in practice?
It's always nice to have numbers in the commit message for patches that
promise performance improvements.

>  hw/scsi/scsi-disk.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> index 8cf50845ab..ce48e20ee6 100644
> --- a/hw/scsi/scsi-disk.c
> +++ b/hw/scsi/scsi-disk.c
> @@ -43,6 +43,7 @@
>  #include "qemu/cutils.h"
>  #include "trace.h"
>  #include "qom/object.h"
> +#include "block/block_int-common.h"
>  
>  #ifdef __linux
>  #include <scsi/sg.h>
> @@ -75,7 +76,7 @@ struct SCSIDiskClass {
>       */
>      DMAIOFunc       *dma_readv;
>      DMAIOFunc       *dma_writev;
> -    bool            (*need_fua_emulation)(SCSICommand *cmd);
> +    bool            (*need_fua)(SCSICommand *cmd);
>      void            (*update_sense)(SCSIRequest *r);
>  };
>  
> @@ -86,6 +87,7 @@ typedef struct SCSIDiskReq {
>      uint32_t sector_count;
>      uint32_t buflen;
>      bool started;
> +    bool need_fua;
>      bool need_fua_emulation;
>      struct iovec iov;
>      QEMUIOVector qiov;
> @@ -553,7 +555,7 @@ static void scsi_read_data(SCSIRequest *req)
>  
>      first = !r->started;
>      r->started = true;
> -    if (first && r->need_fua_emulation) {
> +    if (first && r->need_fua) {
>          block_acct_start(blk_get_stats(s->qdev.conf.blk), &r->acct, 0,
>                           BLOCK_ACCT_FLUSH);
>          r->req.aiocb = blk_aio_flush(s->qdev.conf.blk, scsi_do_read_cb, r);
> @@ -2384,7 +2386,9 @@ static int32_t scsi_disk_dma_command(SCSIRequest *req, uint8_t *buf)
>          scsi_check_condition(r, SENSE_CODE(LBA_OUT_OF_RANGE));
>          return 0;
>      }
> -    r->need_fua_emulation = sdc->need_fua_emulation(&r->req.cmd);
> +    r->need_fua = sdc->need_fua(&r->req.cmd);
> +    r->need_fua_emulation = r->need_fua &&
> +        (blk_bs(s->qdev.conf.blk)->supported_write_flags & BDRV_REQ_FUA) == 0;

You can just use BDRV_REQ_FUA unconditionally. If the driver doesn't
support it directly, the block layer already emulates it internally. We
don't have to duplicate this here. If scsi_write_data() does a flush
directly for VERIFY (like scsi_read_data() already does),
scsi_write_do_fua() can go away completely.

However, we can only apply this to write requests. We still need to know
that FUA needs to be emulated for reads. scsi_read_data() issues a flush
for FUA requests and your patch would break it if writes support
BDRV_REQ_FUA.

Kevin

Re: [PATCH 1/2] scsi-disk: Advertise FUA support by default

[Kevin Wolf]

Am 04.03.2025 um 17:15 hat Daniel P. Berrangé geschrieben:
> On Tue, Mar 04, 2025 at 03:52:31PM +0000, Alberto Faria wrote:
> > FUA emulation code is already is place.
> > 
> > Signed-off-by: Alberto Faria <afaria@redhat.com>
> > ---
> >  hw/scsi/scsi-disk.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> > index e7f738b484..8cf50845ab 100644
> > --- a/hw/scsi/scsi-disk.c
> > +++ b/hw/scsi/scsi-disk.c
> > @@ -3212,7 +3212,7 @@ static const Property scsi_hd_properties[] = {
> >      DEFINE_PROP_BIT("removable", SCSIDiskState, features,
> >                      SCSI_DISK_F_REMOVABLE, false),
> >      DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
> > -                    SCSI_DISK_F_DPOFUA, false),
> > +                    SCSI_DISK_F_DPOFUA, true),
> 
> Should this come with machine type compat to prevent us advertizing FUA
> to existing machine type versions ?

To give a little more background to Alberto: When upgrading QEMU, you
don't want the guest-visible machine to change. You already don't really
want hardware changes while the VM is down (Windows might require
reactivation etc.), but we support live migration between old and new
versions (often in both directions) as long as you use the same machine
type, and no guest expects that the hardware changes while it's running.

So we can only change guest-visible features in new machine types. What
you set here is fine for all new machine types, but you'll have to set
it back to false in hw_compat_9_2 in hw/core/machine.c so that all older
machine types still get the old default.

Kevin

Re: [PATCH 2/2] scsi-disk: Add native FUA support

[Kevin Wolf]

Am 06.03.2025 um 11:33 hat Kevin Wolf geschrieben:
> Am 04.03.2025 um 16:52 hat Alberto Faria geschrieben:
> > Avoid emulating FUA when the driver supports it natively. This should
> > provide better performance than a full flush after the write.
> > 
> > Signed-off-by: Alberto Faria <afaria@redhat.com>
> 
> Did you try out if you can see performance improvements in practice?
> It's always nice to have numbers in the commit message for patches that
> promise performance improvements.

I was curious enough to see how this and the recent series by Stefan
(virtio-scsi multiqueue) and myself (FUA on the backend + polling
improvements) play out with virtio-scsi, so I just ran some fio
benchmarks with sync=1 myself to compare:

iops bs=4k cache=none           |    virtio-scsi    |     virtio-blk    |
O_SYNC workload                 |   qd 1  |  qd 16  |   qd 1  |  qd 16  |
--------------------------------+---------+---------+---------+---------+
master                          |   21296 |  109747 |   25762 |  130576 |
+ virtio-scsi multiqueue        |   28798 |  121170 |       - |       - |
+ FUA in scsi-disk              |   51893 |  204199 |       - |       - |
--------------------------------+---------+---------+---------+---------+
Total change                    | +143.7% |  +86.1% |       - |       - |

(No new numbers for virtio-blk because virtio-scsi patches obviously
don't change anything about it. Also no numbers for FUA in file-posix
because it's unused with cache=none.)

iops bs=4k cache=directsync     |    virtio-scsi    |     virtio-blk    |
O_SYNC workload                 |   qd 1  |  qd 16  |   qd 1  |  qd 16  |
--------------------------------+---------+---------+---------+---------+
master                          |   32223 |  109748 |   45583 |  258416 |
+ FUA in file-posix + polling   |   32148 |  198665 |   58601 |  320190 |
+ virtio-scsi multiqueue        |   51739 |  225031 |       - |       - |
+ FUA in scsi-disk              |   56061 |  227535 |       - |       - |
--------------------------------+---------+---------+---------+---------+
Total change                    |  +74.0% | +107.3% |  +28.6% |  +23.9% |

Of course, the huge improvements on the virtio-scsi side only show how
bad it was before. In most numbers it is still behind virtio-blk even
after all three patch series (apart from cache=none where the
availability of FUA on the device side makes a big difference, and I
expect that virtio-blk will improve similarly once we implement it
there).

Also note that when testing the virtio-scsi multiqueue patches, this
was still a single iothread, i.e. I wasn't even making use of the new
feature per se. I assume much of this comes from enabling polling
because the series moved the event queue handling to the main loop,
which prevented polling for virtio-scsi before. The series also got rid
of an extra coroutine per request for the blk_is_available() call in
virtio_scsi_ctx_check(), which might play a role, too.

Anyway, I like these numbers for FUA in scsi-disk. It makes write back
cache modes almost catch up to write through with O_SYNC workloads. We
should definitely get this merged and do the same for virtio-blk.

Kevin

Re: [PATCH 2/2] scsi-disk: Add native FUA support

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 3502 bytes --]

On Tue, Mar 25, 2025 at 01:48:35PM +0100, Kevin Wolf wrote:
> Am 06.03.2025 um 11:33 hat Kevin Wolf geschrieben:
> > Am 04.03.2025 um 16:52 hat Alberto Faria geschrieben:
> > > Avoid emulating FUA when the driver supports it natively. This should
> > > provide better performance than a full flush after the write.
> > > 
> > > Signed-off-by: Alberto Faria <afaria@redhat.com>
> > 
> > Did you try out if you can see performance improvements in practice?
> > It's always nice to have numbers in the commit message for patches that
> > promise performance improvements.
> 
> I was curious enough to see how this and the recent series by Stefan
> (virtio-scsi multiqueue) and myself (FUA on the backend + polling
> improvements) play out with virtio-scsi, so I just ran some fio
> benchmarks with sync=1 myself to compare:
> 
> iops bs=4k cache=none           |    virtio-scsi    |     virtio-blk    |
> O_SYNC workload                 |   qd 1  |  qd 16  |   qd 1  |  qd 16  |
> --------------------------------+---------+---------+---------+---------+
> master                          |   21296 |  109747 |   25762 |  130576 |
> + virtio-scsi multiqueue        |   28798 |  121170 |       - |       - |
> + FUA in scsi-disk              |   51893 |  204199 |       - |       - |
> --------------------------------+---------+---------+---------+---------+
> Total change                    | +143.7% |  +86.1% |       - |       - |
> 
> (No new numbers for virtio-blk because virtio-scsi patches obviously
> don't change anything about it. Also no numbers for FUA in file-posix
> because it's unused with cache=none.)
> 
> iops bs=4k cache=directsync     |    virtio-scsi    |     virtio-blk    |
> O_SYNC workload                 |   qd 1  |  qd 16  |   qd 1  |  qd 16  |
> --------------------------------+---------+---------+---------+---------+
> master                          |   32223 |  109748 |   45583 |  258416 |
> + FUA in file-posix + polling   |   32148 |  198665 |   58601 |  320190 |
> + virtio-scsi multiqueue        |   51739 |  225031 |       - |       - |
> + FUA in scsi-disk              |   56061 |  227535 |       - |       - |
> --------------------------------+---------+---------+---------+---------+
> Total change                    |  +74.0% | +107.3% |  +28.6% |  +23.9% |
> 
> Of course, the huge improvements on the virtio-scsi side only show how
> bad it was before. In most numbers it is still behind virtio-blk even
> after all three patch series (apart from cache=none where the
> availability of FUA on the device side makes a big difference, and I
> expect that virtio-blk will improve similarly once we implement it
> there).
> 
> Also note that when testing the virtio-scsi multiqueue patches, this
> was still a single iothread, i.e. I wasn't even making use of the new
> feature per se. I assume much of this comes from enabling polling
> because the series moved the event queue handling to the main loop,
> which prevented polling for virtio-scsi before. The series also got rid
> of an extra coroutine per request for the blk_is_available() call in
> virtio_scsi_ctx_check(), which might play a role, too.
> 
> Anyway, I like these numbers for FUA in scsi-disk. It makes write back
> cache modes almost catch up to write through with O_SYNC workloads. We
> should definitely get this merged and do the same for virtio-blk.

Thanks for sharing! Nice IOPS improvements across the board.

Stefan

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