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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.11.13 at 14:57, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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?

Because OSes may make assumptions (whether such assumptions
are universally valid is another thing of course).

Jan


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