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

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

[Paolo Bonzini]

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);
+    }
 
     return l;
 }
-- 
1.8.1.4

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

[Peter Maydell]

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

-- PMM

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

[Paolo Bonzini]

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

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

[Paolo Bonzini]

Il 29/07/2013 14:28, Paolo Bonzini ha scritto:
> 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);
> +    }
>  
>      return l;
>  }
> 

Ping, and adding qemu-stable.

Paolo