Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API

[Christoph Hellwig]

On Sun, Mar 15, 2009 at 04:57:53PM +0200, Avi Kivity wrote:
> Christoph Hellwig wrote:
> >The patch series (now posted) makes all aio APIs use iovecs.  The
> >current non-iovec based SG API would require some nasty shims allocating
> >nested aiocbs.  Or we could just make the sg aio methods fake-ioveced
> >which is the solution I had before your split, always adding a
> >one-element iovec to the scsi-generic request structure.
> >  
> 
> I'd go for having everything vectored.  It's how non-1962 hardware works.

Yeah, that's what I did for regular block I/O in my series.
scsi-generic is a little different beast, though.  qemu uses the legacy
(not sure if it's formally deprecated) read/write support on /dev/sg
which doen't support vectored I/O.  If we want to use vectored I/O
with sg we'd have to use the SG_IO ioctl (which also has the advantage
of working with all sd/sr/ide/etc block devices).  But supporting that
would probably require very different APIs then the current (aio-)
read/writeish one.