On Thu, 2020-12-17 at 18:56 +0200, Maxim Levitsky wrote:
> This patch series attempts to provide a solution to the problem of the
> transfer
> limits of the raw file driver (host_device/file-posix), some of which I
> already tried to fix in the past.
>
> I included 2 patches from Tom Yan which fix two issues with reading the limits
> correctly from the */dev/sg* character devices in the first place.
>
> The only change to these patches is that I tweaked a bit the comments in the
> source to better document the /dev/sg quirks.
>
> The other two patches in this series split the max transfer limits that qemu
> block devices expose in two:
> One limit is for the regular IO, and another is for the SG_IO (aka
> bdrv_*_ioctl),
> and the two device drivers (scsi-block and scsi-generic) that use the later
> are switched to the new interface.
>
> This should ensure that the raw driver can still advertise the unlimited
> transfer length, unless it is used for SG_IO, because that yields the highest
> performance.
>
> Also I include a somewhat unrelated fix to a bug I found in qemu's
> SCSI passthrough while testing this:
> When qemu emulates the VPD block limit page, for a SCSI device that doesn't
> implement it, it doesn't really advertise the emulated page to the guest.
>
> I tested this by doing both regular and SG_IO passthrough of my
> USB SD card reader.
>
> That device turned out to be a perfect device for the task, since it has max
> transfer size of 1024 blocks (512K), and it enforces it.
>
> Also it didn't implement the VPD block limits page,
> (transfer size limit probably comes from something USB related) which
> triggered
> the unrelated bug.
>
> I was able to see IO errors without the patches, and the wrong max transfer
> size in the guest, and with patches both issues were gone.
>
> I also found an unrelated issue in /dev/sg passthrough in the kernel.
> It turns out that in-kernel driver has a limitation of 16 requests in flight,
> regardless of what underlying device supports.
>
> With a large multi-threaded fio job and a debug print in qemu, it is easy to
> see it, although the errors don't do much harm to the guest as it retries the
> IO, and eventually succeed.
> It is an open question if this should be solved.
>
> V2: fixed an issue in a patch from Tom Yan (thanks), and removed
> refactoring from last patch according to Paulo's request.
>
> V3: few cosmitic changes due to the review feedback.
>
> Maxim Levitsky (3):
> block: add max_ioctl_transfer to BlockLimits
> block: use blk_get_max_ioctl_transfer for SCSI passthrough
> block/scsi: correctly emulate the VPD block limits page
>
> Tom Yan (2):
> file-posix: split hdev_refresh_limits from raw_refresh_limits
> file-posix: add sg_get_max_segments that actually works with sg
>
> block/block-backend.c | 12 ++++++
> block/file-posix.c | 76 +++++++++++++++++++++++++++-------
> block/io.c | 2 +
> block/iscsi.c | 1 +
> hw/scsi/scsi-generic.c | 13 ++++--
> include/block/block_int.h | 4 ++
> include/sysemu/block-backend.h | 1 +
> 7 files changed, 90 insertions(+), 19 deletions(-)
>
> --
> 2.26.2
>
>
>
Any update on this?
Best regards,
Maxim Levitsky