Re: [Qemu-devel] [RFC] Writeable files in fw_cfg

[Gleb Natapov]

On Mon, Jan 28, 2013 at 10:10:06AM -0600, Anthony Liguori wrote:
> David Woodhouse <address@hidden> writes:
> 
> > On Sun, 2013-01-27 at 18:53 -0600, Anthony Liguori wrote:
> >> Are you just trying to persist a single blob of a fixed maximum size?
> >
> > That would suffice.
> >
> >> Why not just have a second flash device then?
> >
> > Mostly because flash devices don't actually *work* with KVM.
> 
> They absolutely do.  What doesn't work is executing ROM from flash if
> the ROM cannot be treated as read-only memory.
> 
> That's because all we get is a PF in the kernel when trying to execute
> from unmapped ROM.  There's no way to turn that into MMIO to userspace
> without switching to running fully in emulation mode.  The x86 emulator
> is pretty close to complete but work would be needed to fully complete
> it to make this work.
> 
The x86 emulator cannot fetch from MMIO memory today. And protected mode
emulation is not so complete. We rarely, if ever, need to emulate
protected mode instructions without memory operands.

> We normally handle this by mapping the ROM memory read-only so it can be
> executed without PF'ing but since the BIOS area is subject to PAM, we
> can't use this trick for that particular ROM.
Up until kernel 3.7 there was no support for read-only memory slots. As
far as I know QEMU still maps ROM as regular RAM to KVM.

> 
> SeaBIOS has hack to just not use PAM to do BIOS shadowing when running
> under KVM/QEMU but presumably OVMF lacks this.
Not sure that such hack exists and why is it needed. BIOS area is always
writable in KVM. 

> 
> But in this case, you're using the flash device purely for read/write,
> not for execution, so there's no limitation at all.
> 
Yes, on newer kernel we can use read-only slots to emulate flash. They
behaves like ROMD: memory on read MMIO on write. On older kernels we can
use pure MMIO.

--
                        Gleb.