[Qemu-devel] Re: [PATCH 2/3] Assume PPC64 host on PPC32 KVM

[Jan Kiszka]

Alexander Graf wrote:
> 
> On 24.07.2009, at 13:51, Jan Kiszka wrote:
> 
>> Alexander Graf wrote:
>>>
>>> On 24.07.2009, at 13:17, Jan Kiszka wrote:
>>>
>>>> Alexander Graf wrote:
>>>>>
>>>>> On 24.07.2009, at 12:59, Jan Kiszka wrote:
>>>>>
>>>>>> Alexander Graf wrote:
>>>>>>> When talking to the kernel about dirty maps, we need to find out
>>>>>>> which
>>>>>>> bits were actually set. This is done by set_bit and test_bit like
>>>>>>> functiontality which uses the "long" variable type.
>>>>>>>
>>>>>>> Now, with PPC32 userspace and PPC64 kernel space (which is pretty
>>>>>>> common),
>>>>>>> we can't interpret the bits properly anymore, because we think
>>>>>>> long is
>>>>>>> 32 bits wide.
>>>>>>>
>>>>>>> So for PPC dirty bitmap analysis, let's just assume we're always
>>>>>>> running
>>>>>>> on a PPC64 host. Currently there is no dirty bitmap
>>>>>>> implementation for
>>>>>>> PPC32 / PPCEMB anyways.
>>>>>>>
>>>>>>> Unbreaks dirty logging on PPC.
>>>>>>>
>>>>>>> Signed-off-by: Alexander Graf <address@hidden>
>>>>>>> ---
>>>>>>> kvm-all.c |    6 ++++++
>>>>>>> 1 files changed, 6 insertions(+), 0 deletions(-)
>>>>>>>
>>>>>>> diff --git a/kvm-all.c b/kvm-all.c
>>>>>>> index 824bb4c..bfaa623 100644
>>>>>>> --- a/kvm-all.c
>>>>>>> +++ b/kvm-all.c
>>>>>>> @@ -357,7 +357,13 @@ int
>>>>>>> kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>>>>>>>       for (phys_addr = mem->start_addr, addr = mem->phys_offset;
>>>>>>>            phys_addr < mem->start_addr + mem->memory_size;
>>>>>>>            phys_addr += TARGET_PAGE_SIZE, addr +=
>>>>>>> TARGET_PAGE_SIZE) {
>>>>>>> +#ifdef HOST_PPC
>>>>>>> +             /* Big endian keeps us from having different long
>>>>>>> sizes
>>>>>>> in user and
>>>>>>> +              * kernel space, so assume we're always on ppc64. */
>>>>>>> +            uint64_t *bitmap = (uint64_t *)d.dirty_bitmap;
>>>>>>> +#else
>>>>>>>           unsigned long *bitmap = (unsigned long *)d.dirty_bitmap;
>>>>>>> +#endif
>>>>>>>           unsigned nr = (phys_addr - mem->start_addr) >>
>>>>>>> TARGET_PAGE_BITS;
>>>>>>>           unsigned word = nr / (sizeof(*bitmap) * 8);
>>>>>>>           unsigned bit = nr % (sizeof(*bitmap) * 8);
>>>>>>
>>>>>> This rather screams for a generic fix. Current code assumes
>>>>>> sizeof(unsigned long) == 8. That should already break on 32-bit x86
>>>>>> hosts. So either do (sizeof(*bitmap) * sizeof(unsigned long)) or
>>>>>> switch
>>>>>> to uint64_t - but for ALL hosts.
>>>>>
>>>>> I don't see where that would break. The kernel treats the array as
>>>>> ulong*, userspace treats it as ulong* and set_bit in kernel does
>>>>> bitmap[word] |= (1 << bit). So as long as userspace long and kernel
>>>>> long
>>>>> are the same, it works.
>>>>>
>>>>> In fact - it should even work out with little endian and different
>>>>> ulong
>>>>> sizes. It just breaks on BE.
>>>>
>>>> Err, yes, forget it.
>>>>
>>>> But let's help me understanding the actual problem: Do you have
>>>> different ulong sizes in your scenario? Why? Is it a compat issue of
>>>> 32-bit userland on 64-bit kernel?
>>>
>>> 32-bit userland on 64-bit kernel.
>>
>> OK. So this is an issue due to an underspecified KVM ABI, right?
> 
> Well it's a design decision in the (generic KVM) ABI.

I wouldn't call it a "decision" :). I think it happened to be ulong as
KVM grew up on x86.

> 
>>> kernel: sizeof(ulong) = 8
>>> userspace: sizeof(ulong) = 4
>>>
>>> now, with big endian, a "1" is on the rightmost byte - which means
>>> looking at the bytes it's
>>>
>>> kernel: byte[7]
>>> userspace: byte[3]
>>>
>>> So if you set bit nr "1" with the current logic, the kernel would set
>>> bit "1" (in the first 8 bytes), userspace would read bit "1" in the
>>> second byte, thus 32 + 1.
>>>
>>> On little endian, the lower word is on the first 4 bytes, so it would
>>> still be bit "1" in the first byte.
>>>
>>
>> Big endian machines require us to agree on the word size of the bitmap
>> so that 32-on-64-bit works - and 32-on-32 doesn't break. I think the
>> latter would be the case with your patch, no? Or don't we have 32-bit
>> KVM PowerPC kernels?
> 
> There are no 32-bit PowerPC KVM kernels that can do dirty logging.

By design or by lack of implementation?

And what if some other arch with support for both pops up? #ifdef
HOST_PPC is no long-term solutions. And I see no need to install a
temporary workaround given the currently existing kernel support.

> 
>> In any case, I suggest to pin down the word size and use it for all
>> hosts.
> 
> That would break backwards compatibility.

x86 and ia64 are little endian for which is doesn't matter, PowerPC and
s390 don't support dirty logging so far. Or what would break?

Jan

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