Re: [PATCH 1/2] hw_random: core updates to allow more efficient drivers

[Ian Molton <ian.molton@collabora.co.uk>]

Matt Mackall wrote:
> On Mon, 2009-11-30 at 10:28 +0000, Ian Molton wrote:
>> Rusty Russell wrote:
>>
>>> And might as well just #defube RNG_BUFFSIZE SMP_CACHE_BYTES (or use
>>> SMP_CACHE_BYTES here and sizeof() elsewhere).
>> This can lead to a rather small (4 byte) buffer on some systems, however
>> I don't know if in practice a tiny buffer or a big one would be better
>> for performance on those machines. I guess if its a problem someone can
>> patch the code to allocate a minimum of (say) 16 bytes in future...
> 
> Hmmm, I think this was bad advice from Rusty.

Not entirely...

> The goal is to size and align the buffer so that we know it will always
> work. Thus 64 bytes (always big enough but not so big that anyone will
> complain) and cache aligned (makes stupid things like Via Padlock happy
> -on Vias-). 

yep. Although making it the size of a cacheline makes sense on /most/
modern architectures - 32 bytes is a very common size - I think the
(current) worst case is one of the drivers wants to dump 3 u64s in one
go. virtio-rng will take what it can...

> Rusty's suggestion could easily have us in trouble if some driver wants
> to hand us a mere 64 bits on an architecture with 4-byte cache alignment
> but is otherwise perfectly happy with 64-bit stores. 

How about SNP_CACHE_BYTES or if less, then 32 bytes minimum? Or just
stick with 64 bytes. Either way works for me.

-Ian