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Re: [Qemu-devel] [PATCH v2 0/7] dma-helpers, scsi-block: use SG_IO for a

From: Mark Cave-Ayland
Subject: Re: [Qemu-devel] [PATCH v2 0/7] dma-helpers, scsi-block: use SG_IO for all I/O on scsi-block
Date: Mon, 23 May 2016 20:36:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.8.0

On 23/05/16 13:54, Paolo Bonzini wrote:

> scsi-block uses the block layer for reads and writes in order to avoid
> allocating bounce buffers as big as the transferred data.  We know how
> to split a large transfer to multiple reads and writes, and thus we can
> use scsi-disk.c's existing code to do I/O in multiple chunks (for non-s/g
> SCSI hosts) or through the DMA helpers (for s/g SCSI hosts).
> Unfortunately, this has the side effect of eating the SCSI status except
> in the very few cases where we can convert an errno code back to a SCSI
> status.  It puts a big wrench in persistent reservations support in the
> guest, for example.
> Luckily, splitting a large transfer into multiple SBC commands is just as
> easy, and this is what the last patch does.  It takes the original CDB,
> patches in a modified starting sector and sector count, and executes the
> SCSI command through blk_aio_ioctl.  It is also easy to pass a QEMUIOVector
> to SG_IO, so that s/g SCSI hosts keep the performance.
> This rebases the patches on top of Eric's changes for byte-based
> BlockBackend access and fixes a few bugs I knew about in the RFC.
> Patches 1, 5 and 6 are new.
> Paolo
> Paolo Bonzini (7):
>   dma-helpers: change interface to byte-based
>   dma-helpers: change BlockBackend to opaque value in DMAIOFunc
>   scsi-disk: introduce a common base class
>   scsi-disk: introduce dma_readv and dma_writev
>   scsi-disk: add need_fua_emulation to SCSIDiskClass
>   scsi-disk: introduce scsi_disk_req_check_error
>   scsi-block: always use SG_IO
>  dma-helpers.c        |  54 +++++--
>  hw/block/nvme.c      |   6 +-
>  hw/ide/ahci.c        |   6 +-
>  hw/ide/core.c        |  20 ++-
>  hw/ide/internal.h    |   6 +-
>  hw/ide/macio.c       |   2 +-
>  hw/scsi/scsi-disk.c  | 409 
> +++++++++++++++++++++++++++++++++++++--------------
>  include/sysemu/dma.h |  20 +--
>  trace-events         |   2 +-
>  9 files changed, 371 insertions(+), 154 deletions(-)

Hi Paolo,

I thought I'd give this patchset a spin with a view to seeing whether I
could switch the macio device back to the now byte-based dma-helpers,
but came up with a couple of compile errors. Attached is the minor diff
I applied in order to get a successful compile (apologies for not being
inline, however I couldn't get my mail client to stop wrapping incorrectly).



Attachment: dma-helpers-compile-fix.patch
Description: Text Data

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