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

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/12/2015 09:47, Alex Pyrgiotis wrote:
>> > 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.

Right, I forgot about that.  However, falling back to scsi-generic
effectively means that scsi-block is always O_DIRECT/cache=none.  So why
not just specify cache=none?

We can improve the code to print a warning if you don't.  (It needs some
care: iscsi never caches, independent of the cache= argument, so we
don't want to warn for it.  But it can be done).

Paolo