[OpenBIOS] [PATCH] ppc: fix reuse of MMU translation slots

[Alexander Graf]

On 02/12/2016 12:38 PM, Mark Cave-Ayland wrote:
> On 10/02/16 23:25, Alyssa Milburn wrote:
>
>> The current code always picks slot 0 in the reuse case. (This doesn't seem
>> to matter in the cases I've tested, though.)
>>
>> Signed-off-by: Alyssa Milburn <fuzzie at fuzzie.org>
>> ---
>>   arch/ppc/qemu/ofmem.c | 16 ++++++++++------
>>   1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/ppc/qemu/ofmem.c b/arch/ppc/qemu/ofmem.c
>> index 6f346a3..c9fe154 100644
>> --- a/arch/ppc/qemu/ofmem.c
>> +++ b/arch/ppc/qemu/ofmem.c
>> @@ -359,9 +359,11 @@ hash_page_64(unsigned long ea, phys_addr_t phys, ucell mode)
>>               found = 1;
>>   
>>       /* otherwise use a free slot */
>> -    for (i = 0; !found && i < 8; i++)
>> -        if (!pp[i].v)
>> -            found = 1;
>> +    if (!found) {
>> +        for (i = 0; !found && i < 8; i++)
>> +            if (!pp[i].v)
>> +                found = 1;
>> +    }
>>   
>>       /* out of slots, just evict one */
>>       if (!found)
>> @@ -414,9 +416,11 @@ hash_page_32(unsigned long ea, phys_addr_t phys, ucell mode)
>>               found = 1;
>>   
>>       /* otherwise use a free slot */
>> -    for (i = 0; !found && i < 8; i++)
>> -        if (!pp[i].v)
>> -            found = 1;
>> +    if (!found) {
>> +        for (i = 0; !found && i < 8; i++)
>> +            if (!pp[i].v)
>> +                found = 1;
>> +    }
>>   
>>       /* out of slots, just evict one */
>>       if (!found)
>>
> Hmmm yes I think you're right there is a bug here. Assuming that we
> locate the index of an old translation, then i should be set to the slot
> number to replace. But even though found has been set, the subsequent
> for() will always reset i to zero, even though it exits without iterating.
>
> In theory this could mean that we have multiple entries in the hash
> table for the same translation which can't be a good thing.

Yes, on real hardware machine checks can result from this.

>
> Alex - does this look like the right solution to you?

Looks reasonable to me.

Alex