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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 13:48:01 +0200

On Thu, Mar 10, 2011 at 12:27:55PM +0100, Jan Kiszka wrote:
> 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.
It is not much different from what I proposed. The result will be the
same. Even option syntax will probably be the same :)

> > 
> > 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?
Yes we can make memory slot that will be treated as memory on read and
IO on write, but first relying on that will prevent using flash interface
on older kernels and second it is not enough to implement the proposal.
When magic value is written into an address, the address become IO for
reading too, but KVM slot granularity is page, not byte, so KVM will
have to remove the slot to make it IO, but KVM can't execute code from
IO region (yet), so we will not be able to run firmware from flash and
simultaneously write into the flash. 

>                                                                 Virtio
> means that you have to patch the guest (which might be something else
> than flexible Linux...).
This intended to be used by firmware only and we control that.


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