Re: [Xen-devel] [PATCH] hvmloader: write extra memory in CMOS

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Nov 12, 2013 at 12:30:52PM +0000, Jan Beulich wrote:
> >>> On 12.11.13 at 13:11, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> >>As it stands for HVM guests the e820 map is determined by hvmloader, so
> >>it makes sense for it to populate standard CMOS locations with the
> >>values they should have.
> > 
> > CMOS has memory values? That is standard PC spec? Yikes!
> 
> That's the first time I hear about this - there are a couple of more
> or less standard locations in CMOS where some of the memory
> gets reported, but all are at most two bytes wide and (having at
> best 64k granularity) don't allow expressing memory beyond 4Gb.
> 
> Now, if there is a standard for the locations used here, fine with
> me (but it should be referenced in the commit message then). But
> if this is custom, then I wonder (a) how compatible such an
> extension is going to be and (b) why it needs to be restricted to
> 3 bytes (allowing to cover only up to 1Tb).
> 

It's not custom.

AFAICT Boches reads this, seabios reads this (when not running on Xen)
and OVMF reads this as well.

http://bochs.sourceforge.net/doc/docbook/development/cmos-map.html

However in some CMOS maps I found those bytes are marked as reserved.
That's why I say "Some BIOSes" in comment.

http://www.bioscentral.com/misc/cmosmap.htm

Wei.

> Jan

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