Re: [PATCH 0/5] SCSI: fix transfer limits for SCSI passthrough

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2020-11-04 at 19:32 +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.
> 
> 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             | 79 +++++++++++++++++++++++++++-------
>  block/io.c                     |  2 +
>  block/iscsi.c                  |  1 +
>  hw/scsi/scsi-generic.c         | 32 ++++++++------
>  include/block/block_int.h      |  4 ++
>  include/sysemu/block-backend.h |  1 +
>  7 files changed, 103 insertions(+), 28 deletions(-)
> 
> -- 
> 2.26.2
> 
Very gentle ping on this path series.

Best regards,
	Maxim Levitsky