Re: [Qemu-devel] [PATCH] x86: only allow real mode to access 32bit without LMA

[Stefan Weil]

Am 06.12.2013 19:48, schrieb Michael Tokarev:
> 06.12.2013 16:52, Alexander Graf wrote:
>> When we're running in non-64bit mode with qemu-system-x86_64 we can
>> still end up with virtual addresses that are above the 32bit boundary
>> if a segment offset is set up.
>>
>> GNU Hurd does exactly that. It sets the segment offset to 0x80000000 and
>> puts its EIP value to 0x8xxxxxxx to access low memory.
>>
>> This doesn't hit us when we enable paging, as there we just mask away the
>> unused bits. But with real mode, we assume that vaddr == paddr which is
>> wrong in this case. Real hardware wraps the virtual address around at the
>> 32bit boundary. So let's do the same.
>>
>> This fixes booting GNU Hurd in qemu-system-x86_64 for me.
>>
>> Reported-by: Michael Tokarev <address@hidden>
>> Signed-off-by: Alexander Graf <address@hidden>
>> ---
>>  target-i386/helper.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/target-i386/helper.c b/target-i386/helper.c
>> index 7c196ff..ed965d6 100644
>> --- a/target-i386/helper.c
>> +++ b/target-i386/helper.c
>> @@ -531,6 +531,12 @@ int cpu_x86_handle_mmu_fault(CPUX86State *env, 
>> target_ulong addr,
>>  
>>      if (!(env->cr[0] & CR0_PG_MASK)) {
>>          pte = addr;
>> +#ifdef TARGET_X86_64
>> +        if (!(env->hflags & HF_LMA_MASK)) {
>> +            /* Without long mode we can only address 32bits in real mode */
>> +            pte = (uint32_t)pte;
>> +        }
>> +#endif
>>          virt_addr = addr & TARGET_PAGE_MASK;
>>          prot = PAGE_READ | PAGE_WRITE | PAGE_EXEC;
>>          page_size = 4096;
>
> Tested-by: Michael Tokarev <address@hidden>
>
> Well, it isn't much of testing, I too just run hurd image and see that
> it can work in qemu-x86_64 tcg mode, while without this patch it
> segfaults.
>
> Should I apply this to trivial queue? :)
>
> Thanks,
>
> /mjt

Maybe your trivial queue would be the fasted way to get it into the
official repository. :-)

I added qemu-stable to the addressees, because this is useful for the
stable branches as well.

Regards,

Stefan