Re: [Qemu-devel] [PATCH 3/3] support readonly memory feature in qemu

[Kevin O'Connor]

On Mon, Sep 10, 2012 at 11:25:38AM +0200, Jan Kiszka wrote:
> On 2012-09-09 17:45, Avi Kivity wrote:
> > On 09/07/2012 11:50 AM, Jan Kiszka wrote:
> >>
> >>> +            } else {
> >>> +                cpu_physical_memory_rw(run->mmio.phys_addr,
> >>> +                                       run->mmio.data,
> >>> +                                       run->mmio.len,
> >>> +                                       run->mmio.is_write);
> >>> +            }
> >>> +
> >>>              ret = 0;
> >>>              break;
> >>>          case KVM_EXIT_IRQ_WINDOW_OPEN:
> >>>
> >>
> >> Great to see this feature for KVM finally! I'm just afraid that this
> >> will finally break good old isapc - due to broken Seabios. KVM used to
> >> "unbreak" it as it didn't respect write protections. ;)
> > 
> > Can you describe the breakage?
> 
> Try "qemu -machine isapc [-enable-kvm]". Seabios is writing to some
> read-only marked area. Don't recall where precisely.

On boot, QEMU marks the memory at 0xc0000-0x100000 as read-only.
SeaBIOS then makes the area read-write, performs its init, and then
makes portions of it read-only before launching the OS.

The registers SeaBIOS uses to make the memory read-write are on a PCI
device.  On isapc, this device is not reachable, and thus SeaBIOS
can't make the memory writable.

The easiest way to fix this is to change QEMU to boot with the area
read-write.  There's no real gain in booting with the memory read-only
as the first thing SeaBIOS does is make it read-write.

-Kevin