Re: [Qemu-devel] [PATCH 0/9] Add full scatter-gather support for SCSI generic devices

[Paolo Bonzini]

On 16/12/2015 17:55, Alex Pyrgiotis wrote:
> Hi all,
> 
> This patch is an attempt to boost the performance of "scsi-generic" and
> "scsi-block" device types, by removing an extra data copy and reducing
> their memory footprint. More specifically, the problem lies in the
> functions in the `scsi-generic_req_ops` struct of scsi-generic.c. These
> functions rely on an intermediate buffer to do the SG_IO ioctl request,
> without checking if the SCSI controller has provided a scatter-gather
> list with the request.
> 
> In a nutshell, our proposal is to map the provided scatter-gather list
> (if any) to the address space of the QEMU process and use the resulting
> iovec as the buffer for the ioctl request. You'll find that the logic is
> quite similar to the one used in scsi-disk.c.

Which commands have large payloads and are on the data path, for
scsi-block?  Or is the use case just scsi-generic (e.g. tape devices?)?

(Just trying to understand before I dive into the patches).

Paolo