Re: [Qemu-devel] [PATCH v2 3/5] Qemu: do not mark bios readonly

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

Jan,

On 10/26/2012 06:35 PM, Jan Kiszka wrote:

> This has two problems: We know it breaks at least Win 95 that overwrites
> its F-segment during boot. And it applies changes to the shadowed area
> (below 1 MB) also to the ROM area - I don't think that is the original
> behaviour on real hardware.

So what is the problem? It can break Win95's running?

I tried to install win95 guest but it failed to boot regardless my patchset
was applied or not. I found the information that win 95 is not supported at
http://www.linux-kvm.org/page/Guest_Support_Status

Note: before my patchset, Win 95 still can happily something into ROM area
because readonly memory is actually writable on KVM. And win95 can not run
on isapc with --no-kvm since it is no way to enable shadow ROM.

> 
> What we need is paravirtual shadow write control for the ISA PC. It's on
> my todo list, maybe I will be able to look into this during the next week.
> 

You idea is that modify the code of seabios and use a special way (PV) to
notify Qemu to make the bios writable?

Actually, I am confused why the guest (including bios) persistently uses
shadow ROM even if it is not supported (on ISA PC), i think the right way
is move itself to RAM under this case, no?

> BTW, your patch series should allow to drop the KVM special case from
> pc_system_firmware_init. That version, btw, treats high and low BIOS
> areas separately - but only reloads the upper area. Hmm...
> 

You mean that also allow Qemu to use pflash to load bios if kvm is enabled?
We can not do that for pflash is a RD device which can not be directly written,
kvm can not emulate the instruction which implicitly write the memory. (e.g:
using this area as stack).

Thanks!