Re: [Qemu-devel] [PATCH for-1.6] exec: fix writing to MMIO area with non-power-of-two length

[Paolo Bonzini <pbonzini@redhat.com>]

Il 29/07/2013 14:41, Peter Maydell ha scritto:
> On 29 July 2013 13:28, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> The problem is introduced by commit 2332616 (exec: Support 64-bit
>> operations in address_space_rw, 2013-07-08).  Before that commit,
>> memory_access_size would only return 1/2/4.
>>
>> Since alignment is already handled above, reduce l to the largest
>> power of two that is smaller than l.
>>
>> Reported-by: Oleksii Shevchuk <alxchk@gmail.com>
>> Tested-by: Oleksii Shevchuk <alxchk@gmail.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  exec.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/exec.c b/exec.c
>> index c4f2894..e122c81 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -1925,6 +1925,9 @@ static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
>>      if (l > access_size_max) {
>>          l = access_size_max;
>>      }
>> +    if (l & (l - 1)) {
>> +        l = 1 << (qemu_fls(l) - 1);
>> +    }
> 
> Is this a hot enough code path that we care about going via
> the not-inline qemu_fls() rather than calling clz32() directly?

It is not that hot because of the "if".

> (probably not, I guess.) Alternatively, we seem to have a
> pow2floor() function...

Yeah, pow2floor is also nice.  There's still a lot of opportunity for
unification...

Paolo