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

[Jan Kiszka]

On 2011-03-10 10:47, Gleb Natapov wrote:
> 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.

...or define -flash in a way that allows mapping the bios image as an
overlay to the otherwise guest-managed flash image.

> 
> 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.

Why not enhancing KVM memory slots to support direct read access while
writes are trapped and forwarded to a user space device model? Virtio
means that you have to patch the guest (which might be something else
than flexible Linux...).

Jan

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