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Re: [Qemu-devel] OVMF, SeaBIOS & non-CSM based legacy boot

From: Gleb Natapov
Subject: Re: [Qemu-devel] OVMF, SeaBIOS & non-CSM based legacy boot
Date: Thu, 24 Mar 2011 20:36:38 +0200

On Thu, Mar 24, 2011 at 09:46:09AM -0700, Jordan Justen wrote:
> 2011/3/24 Gleb Natapov <address@hidden>:
> > On Wed, Mar 23, 2011 at 03:32:41PM -0700, Jordan Justen wrote:
> >> By the way, today OVMF attempts to store NV-Var data in a file on the
> >> disk, but this cannot support variables at runtime.  (This is why I
> >> sent in the patch for using -pflash on x86/x86-64.)
> >>
> > And this file is stored always at the same location? If it is then then
> > problem is solved! But what do you mean by "this cannot support
> > variables at runtime"?
> The variables can be set while the OS is running, and the OS has
> exclusive control over the disk at that time.  Today in OVMF we set
> variables into memory during this time, and hope that memory it still
> around after a reset.  This does not provide realistic non-volatile
> UEFI variable support.
KVM preserve memory during reset, but we better not rely on that.

> What we really need is flash memory.  (See my 'hw/pc: Support system
> flash memory' patch.)
Storing boot file only on flash memory will require to distribute the
flash image along with disk image.

> But, there is nothing stopping us from also storing the variables on
> the disk (during the firmware boot), and also using them as a backup.
This will still require at least one reboot for variables to be saved in
a filesystem, but this is better then nothing.

> Additionally, we can add yet another backup system of looking for
> known os-loader executable paths.  This would be needed if a disk
> image were ever to be transferred from a real machine to a VM image.
> But, this would require firmware updates as new UEFI OS loader install
> paths are added.  Also, let's hope no OS decides to generate a random
> path for the OS loader. :)
Firmware updates in a VM is very easy, so not a big deal.


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