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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2021-06-08 at 15:16 +0200, Paolo Bonzini wrote:
> 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;

Actually I think that both max transfer size and max segement count,
are only relevant for SCSI passthrough since kernel I think emualates
both for regular I/O, so I think that we shoudn't expose them to qemu at all.

In my version of the patches I removed both bl.max_transfer and bl.max_iov
setup from the file-posix driver and replaced it with bs->bl.max_ioctl_transfer
(you call it max_hw_transfer)

In my version the bl.max_ioctl_transfer is a merged limit of the max transfer size
and the max iovec number.

https://www.mail-archive.com/qemu-devel@nongnu.org/msg768264.html


Best regards,
	Maxim Levitsky


>          }
>      }
>  
> 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],