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 08:36, Paolo Bonzini wrote:
> 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.

Right, this belongs in the "if (env->hflags & HF_LMA_MASK)" block.

And the subject or description should mention that
x86_cpu_get_phys_page_debug was lacking support for 1G hugepages.

Jan

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