Re: [Qemu-devel] scsi-generic and max request size

[Benjamin Herrenschmidt]

On Thu, 2010-12-23 at 00:23 +0100, Alexander Graf wrote:
> > The non working compat ioctl is one, the fact that "sg" has
> > no /sys/class/block (or /sys/block) entries is another, etc... Ie,
> we
> > are faced with a problem with Linux not exposing those informations
> in
> > an easy to retrieve way, and no proper cross-platform way to obtain
> > those informations neither.
> 
> Why would you care about cross-platform here? Not saying I fully
> understand what information exactly you're lacking. But it's either
> SG_IO max request size in which case you don't need any equivalent on
> other platforms, as it's not available anywhere else. Or it's
> something else in which case you can just set it to some "safe" small
> default value and call it a day :).

Well, do we support something like scsi-generic on windows or BSD
hosts ? dunno.. .just asking :-) They -have- mechanisms (at least
windows do) to pass SCSI requests down the stack. In that case, they'll
have similar limitations (at the very least the max request size).

So we'd want some way to expose that... but if scsi-generic today is
linux only, then I can try to add linux-isms in there as a stop-gap to
try to at least retreive the max req. size which is the main issue for
me right now... at least until I start trying to have the SG_IO
read/write directly into guest memory without bouncing :-) At that
point, the SG limits might become trouble as well.

Cheers,
Ben.