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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-11-12 at 13:21 +0000, Jan Beulich wrote:
> >>> On 12.11.13 at 13:37, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> > 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.
> 
> These are all non-traditional BIOSes, and a reasonably reliable
> documentation aspect can't be taken from their sources or
> accompanying documentation.
> 
> > However in some CMOS maps I found those bytes are marked as reserved.
> 
> Exactly. The question is whether some _other_ BIOSes then use
> these register for other purposes...

Why do we care about any BIOS other than ROMBIOS, SeaBIOS and OVMF?

Ian.



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