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

[Jan Kiszka <jan.kiszka@siemens.com>]

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 <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;
> 

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