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

[Paolo Bonzini <pbonzini@redhat.com>]

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 <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

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