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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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...

Jan


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