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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 18 Mar 2014 08:19:52 +0100
Jan Kiszka <jan.kiszka@siemens.com> wrote:

> 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?

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 <lcapitulino@redhat.com>
> > ---
> > 
> > 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
> > 
> >  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?
> 
> I'll check this more carefully later.
> 
> Jan
> 
> > +
> >          pde_addr = ((pdpe & ~0xfff & ~(PG_NX_MASK | PG_HI_USER_MASK)) +
> >                      (((addr >> 21) & 0x1ff) << 3)) & env->a20_mask;
> >          pde = ldq_phys(cs->as, pde_addr);
> > @@ -993,6 +1000,7 @@ hwaddr x86_cpu_get_phys_page_debug(CPUState *cs, vaddr addr)
> >          pte = pte & env->a20_mask;
> >      }
> >  
> > +out:
> >      page_offset = (addr & TARGET_PAGE_MASK) & (page_size - 1);
> >      paddr = (pte & TARGET_PAGE_MASK) + page_offset;
> >      return paddr;
> > 
>