[Xen-devel] Re: [PATCH] xen: avoid tracking the region 0xa0000 - 0xbffff

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 14 Jun 2011, Alexander Graf wrote:
> On 03.06.2011, at 17:56, <stefano.stabellini@xxxxxxxxxxxxx> 
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 
> > From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > 
> > Xen can only do dirty bit tracking for one memory region, so we should
> > explicitly avoid trying to track the legacy VGA region between 0xa0000
> > and 0xbffff, rather than trying and failing.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > ---
> > xen-all.c |    4 ++++
> > 1 files changed, 4 insertions(+), 0 deletions(-)
> > 
> > diff --git a/xen-all.c b/xen-all.c
> > index 9a5c3ec..1fdc2e8 100644
> > --- a/xen-all.c
> > +++ b/xen-all.c
> > @@ -218,6 +218,10 @@ static int xen_add_to_physmap(XenIOState *state,
> >     if (get_physmapping(state, start_addr, size)) {
> >         return 0;
> >     }
> > +    /* do not try to map legacy VGA memory */
> > +    if (start_addr >= 0xa0000 && start_addr + size <= 0xbffff) {
> 
> I don't quite like the hardcoded range here. What exactly is the issue? The 
> fact that you can only map a single region? Then do a counter and fail when 
> it's > 1. 

That is what we were doing before: succeeding the first time and
failing from the second time on.
By "coincidence" the second time was the range 0xa0000-0xbffff so
everything worked as expected, but it wasn't obvious why.
I am just trying to make sure that one year from now it will be clear
just looking at the code why it works.


> If you don't want to map the VGA region as memory slot, why not change the 
> actual mapping code in the cirrus adapter?

Because I didn't want to introduce any ugly if (xen_enable()) in generic
code, if it is that simple to catch the issue from xen specific code.

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