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[Qemu-devel] Re: RFC: emulation of system flash

From: Gleb Natapov
Subject: [Qemu-devel] Re: RFC: emulation of system flash
Date: Thu, 10 Mar 2011 11:47:26 +0200

On Wed, Mar 09, 2011 at 08:51:23PM -0800, Jordan Justen wrote:
> Hi all,
> I have documented a simple flash-like device which I think could be
> useful for qemu/kvm in some cases.  (Particularly for allowing
> persistent UEFI non-volatile variables.)
> http://wiki.qemu.org/Features/System_Flash
> Let me know if you have any suggestions or concerns.

Two things. First You suggest to replace -bios with -flash. This will
make firmware upgrade painful process that will have to be performed
from inside the guest since the same flash image will contain both
firmware and whatever data was stored on a flash which presumably you
want to reuse after upgrading a firmware. My suggestion is to extend
-bios option like this:

-bios bios.bin,flash=flash.bin,flash_base=addr

flash.bin will be mapped at address flash_base, or, if flash_base is not
present, just below bios.bin.

Second. I asked how flash is programmed because interfaces like CFI
where you write into flash memory address range to issue commands cannot
be emulated efficiently in KVM. KVM supports either regular memory slots
or IO memory, but in your proposal the same memory behaves as IO on
write and regular memory on read. Better idea would be to present
non-volatile flash as ISA virtio device. Should be simple to implement.

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