Re: [Qemu-devel] [PATCH for-2.0?] target-i386: fix gdb debugging with large memory guests

[Paolo Bonzini]

Il 18/03/2014 08:19, Jan Kiszka ha scritto:
> On 2014-03-18 02:54, Luiz Capitulino wrote:
> > If you start a Linux guest with more than 4GB of memory and try to look at a
> > memory address, you will get an error from gdb:
> >
> > (gdb) p node_data[0]->node_id
> > Cannot access memory at address 0xffff88013fffd3a0
> > (gdb)
>
> I suppose this is x86-64, not 32-bit with PTE, right?
>
> > I debugged this down to x86_cpu_get_phys_page_debug(), it doesn't handle the
> > case where the PDPTE has the PS bit set (although I didn't check where Linux
> > sets that bit). This commit adds the PS bit handling, which fixes the problem
> > for me.
> >
> > Signed-off-by: Luiz capitulino <address@hidden>
> > ---
> >
> > Two observations:
> >
> >  1. This bug has always existed, so it's not a regression, so I'm not sure
> >     it's worth it to fix for 2.0

Sure, why not?

> >  2. I'm not familiar with every detail of x86_cpu_get_phys_page_debug(),
> >     so I'm not completely sure this is the right thing to do
> >
> >  target-i386/helper.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/target-i386/helper.c b/target-i386/helper.c
> > index 4f447b8..9b7803f 100644
> > --- a/target-i386/helper.c
> > +++ b/target-i386/helper.c
> > @@ -951,6 +951,13 @@ hwaddr x86_cpu_get_phys_page_debug(CPUState *cs, vaddr 
> > addr)
> >                  return -1;
> >          }
> >
> > +        if (pdpe & PG_PSE_MASK) {
> > +            page_size = 1024 * 1024 * 1024;
> > +            pte = pdpe & ~( (page_size - 1) & ~0xfff);
> > +            pte &= ~(PG_NX_MASK | PG_HI_USER_MASK);
> > +            goto out;
> > +        }
>
> Does this also apply if we are not in long mode?

No, it doesn't.  The only valid bits in a PAE PDPTE are P, PWT and PCD. 
 Bit 7 (PS) is reserved.

Paolo