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

[Alex Pyrgiotis]

Hi Paolo,

On 12/16/2015 08:16 PM, Paolo Bonzini wrote:
> 
> 
> 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).

Sure, no problem. The commands that have large payloads and are on the
data path are the classic SCSI READ/WRITE commands. Usually, these
commands are implemented with vectored reads/writes, which utilize the
controller's scatter-gather list.

However, when opening a "scsi-block" device with the default cache
policy (cache=writeback), QEMU fallbacks to the "scsi-generic" functions
(i.e, SG_IO ioctl requests) for reading/writing data [1]. In this case,
the data are copied in a bounce buffer, which is the issue that this
patch tackles.

Thanks,
Alex


[1]: I'll quote the comment on the code for the rationale behind this
choice:

        "If we are not using O_DIRECT, we might read stale data from
        the host cache if writes were made using other commands than    
        these ones (such as WRITE SAME or EXTENDED COPY, etc.).  So,
        without O_DIRECT everything must go through SG_IO."