[SeaBIOS] [PATCH] fix two issues with virtio-blk

[Gleb Natapov]

Adding Christoph to CC who know much more then me about how block layer
works in qemu and linux.

On Sun, May 16, 2010 at 11:17:42AM -0400, Kevin O'Connor wrote:
> On Sun, May 16, 2010 at 11:08:57AM +0300, Gleb Natapov wrote:
> > On Sat, May 15, 2010 at 05:17:31PM +0200, Sebastian Herbszt wrote:
> > > What the BIOS should do or not doesn't seem to be defined anywhere (yet).
> > > Current drives with physical sector size > 512 bytes report a logical sector
> > > size of 512 bytes and do the emulation on their own. The BIOS doesn't need
> > > to be modified so far to support those drives. If drives with logical sector
> > > size > 512 bytes appear this situation might change. Doing a read emulation
> > Isn't it chicken n' egg situation. If BIOSes will not add 4K block support
> > drives will never be able to use 4K logical blocks.
> 
> It was my understanding that new drives will start up in an emulation
> mode which uses 512 byte sectors.  Later on the OS can activate the
> native sector size.
> 
I am not sure disk can change its topology like that. The meaning of LBA
will change depending on what mode the disk is currently in.

> This also raises a possibility for virtio - maybe a new command or
> flag could be added to temporarily support 512 byte reads.  Thus the
> main OS wont be penalized, and the bios wont have to emulate the reads
> (the host will do it instead).  That said, I'm not against a patch
> which generalizes the SeaBIOS cdemu block conversion code for virtio
> if you wish to do that.  (It will reserve more memory, but I doubt
> anyone using virtio will care.)
> 
As far as I understand if physical disk qemu image resides on has 4K
logical block size we cannot access it from inside a guest using 512 byte
i/o (disk image is opened with O_DIRECT and each i/o has to be
sector size aligned). So even if theoretically emulating 512 byte access
on top of 4K physical disk sector size inside virtio-blk in qemu is possible
it looks more complicated then just switching logical block size during
VM runtime. Christoph what do you think?

--
			Gleb.