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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-11-12 at 14:12 +0000, Jan Beulich wrote:
> >>> 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).

It seems like the use of these CMOS values is a qemu-ism, perhaps
inherited from bochs or somewhere like that, from whence some of the
early virtual BIOSes came. See hw/i386/pc.c:pc_cmos_init().

I think given that it is used in this preexisting way under other virt
platforms there's no cause to worry unduly about what OSes might do.

Ian.


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