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

[Sebastian Herbszt]

Kevin O'Connor wrote:
> On Sat, May 15, 2010 at 05:17:31PM +0200, Sebastian Herbszt wrote:
>>Doing a read emulation
>> in BIOS shouldn't be too bad, but a write emulation will need a "physical
>> sector size" RMW buffer (no clue where the BIOS could steal those 4KB from).
> 
> SeaBIOS has a "malloc_low" function which obtains permanent memory in
> the first 1Meg.  Currently, it grabs memory at the end of 640K (it
> moves the ebda down if needed).  It's probably possible to use memory
> in the c/d/e/f segments - though that's tricky because parts of that
> area are likely to be read-only.
> 
> Right now SeaBIOS allocates a 2K buffer to support cdrom emulation
> (this is different from what bochs bios does).  So, going to a 4K
> buffer is probably reasonable.
> 
> -Kevin

This explains why i see 2 KB less free memory with SeaBIOS.
With current SeaBIOS i get 511 KB free memory running MS-DOS 6.22
with CDROM support. Sure you can grab another 4 KB for a RMW buffer,
but sooner or later there won't be enough memory free to play your favorite
game from the 90s. And i didn't even load an option ROM which also wants
1-2 KB EBDA or some useful TSR program.

Why is a 4 KB physical sector size needed in a VM anyway?

Sebastian