Re: [Xen-devel] Bug: Limitation of <=2GB RAM in domU persists with 4.3.0

[Gordan Bobic <gordan@xxxxxxxxxx>]

On Fri, 26 Jul 2013 10:28:12 +0100, Gordan Bobic <gordan@xxxxxxxxxx> 
wrote:
> On Thu, 25 Jul 2013 21:15:10 -0400, Andrew Bobulsky
> <rulerof@xxxxxxxxx> wrote:
> > On Thu, Jul 25, 2013 at 8:21 PM, Ian Campbell
> > <ian.campbell@xxxxxxxxxx> wrote:
> > > On Thu, 2013-07-25 at 23:23 +0100, Gordan Bobic wrote:
> > > > Now, if I am understanding the basic nature of the problem 
> > > > correctly,
> > > > this _could_ be worked around by ensuring that vBAR = pBAR since 
> > > > in that
> > > > case there is no room for the mis-mapped memory overwrites to 
> > > > occur. Is
> > > > that correct?
> > > AIUI (which is not very well...) it's not so much vBAR=pBAR but 
> > > making
> > > the guest e820 (memory map) have the same MMIO holes as the host so 
> > > that
> > > there can't be any clash between v- or p-BAR and RAM in the guest.
> > >
> > > > I guess I could test this easily enough by applying the vBAR = 
> > > > pBAR hack.
> > > Does the e820_host=1 option help? That might be PV only though, I 
> > > can't
> > > remember...
> > Alas, yes.  The man pages list it under "PV Guest Specific Options":
> > http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html
> Now that is intereting - if this makes the memory holes the same 
> between
> the guest and the host, does it also implicitly vBAR=pBAR?
Another thing that occurred to me might be useful to check - it is
pretty easy to modify the BAR size on Nvidia cards. The defaults are
64MB and 128MB for the two BARs. They can be made much, much larger,
and there is often advantage to enlarging them to at least be equal to
VRAM size. Soooooo... If I boost the BAR from 128MB to 2GB, being a
64-bit BAR, it might make the BIOS do the sane thing and map it above
4GB. With the other BAR also suitably enlarged and it being done on
the second GPU as well, there is no obvious option but to map them
above 4GB (unless the BIOS is broken, which it may well be, in
which case all bets are off).

Which may just alleviate the memory issue if not completely fix
the problem.

Will try this and see what happens.

Gordan

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