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Re: [Qemu-devel] OVMF Google Summer of Code ideas

From: Jordan Justen
Subject: Re: [Qemu-devel] OVMF Google Summer of Code ideas
Date: Tue, 8 Mar 2011 09:13:38 -0800

2011/3/8 Gleb Natapov <address@hidden>:
> On Tue, Mar 08, 2011 at 07:18:09AM +0000, Stefan Hajnoczi wrote:
>> > Regarding the non-volatile variables issue, I have been trying to
>> > develop a proposal for addressing this with a change to QEMU's
>> > hardware support of bios.bin.  But, I don't have the suggestion (or
>> > implementation) ready at this time.
>> Sounds like something to keep discussing with the QEMU and SeaBIOS
>> communities.  Gleb Natapov and Kevin O'Connor have done a lot of the
>> recent BIOS and firmware interface work.  I think persistent CMOS has
>> come up several times and might be similar to non-volatile UEFI
>> storage.
> What kind of information OVMF stores on a persistent storage?

Non-volatile variables are a general system wide environment variable
storage facility, but one key thing to store (for instance) is the
path to the boot image.

> memory is less them 512 byte IIRC and this may not be enough. What OVMF
> uses on real HW for non-volatile storage?

Regarding CMOS, I think qemu exposes 128 bytes of non persistent CMOS
RAM.  Many current chipsets provide another bank of 128 bytes, meaning
a total of 256 bytes (minus the RTC registers).

Regarding UEFI non-volatile variables on real HW:
Most systems today have at least 1MB of flash storage located just
below 4GB.  The entire contents can be modified, which is how firmware
updates happen.

For a UEFI based system, the non-volatile variables generally occupy
8KB~64KB of the flash depending on flash space availability.


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