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

[Alexander Graf]

On 23.12.2010, at 00:35, Benjamin Herrenschmidt wrote:

> 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.

This all belongs in the block layer. If you create a call back function or 
property in the block struct, windows can implement its own limits when someone 
sits down to implement SG_IO on Windows.


Alex