On Tue, Dec 02, 2014 at 03:28:17PM -0500, George Spelvin wrote:
> >> --- a/crypto/ansi_cprng.c
> >> +++ b/crypto/ansi_cprng.c
> >> @@ -51,9 +51,9 @@ struct prng_context {
> >> unsigned char rand_data[DEFAULT_BLK_SZ];
> >> unsigned char DT[DEFAULT_BLK_SZ];
> >> unsigned char V[DEFAULT_BLK_SZ];
> >> - u32 rand_read_pos; /* Offset into rand_data[] */
> >> + unsigned char rand_read_pos; /* Offset into rand_data[] */
>
> > u8 please. Also, not sure if this helps much, as I think the padding
> > will just get you back to word alignment on each of these.
>
> I noticed the "unsigned char" vs "u8" issue, but didn't make the change
> as I didn't think the readailility improvement was worth the code churn.
>
You just sent a 17 patch series of clean up and were worried about code churn
converting an unsigned char to a u8?
> But I'd be happy to clean that up, too!
>
> Should I convert all the buffers and function prototypes, or is there
> some criterion for distinguishing which gets which? (E.g. buffers are
> "unsigned char" but control variables are "u8".)
>
If you want to sure. u8 probably makes more sense for the buffers here as our
intent is to treat them as an array of byte values.
> And actually, you do win. spinlock_t is 16 bits on x86,
> and the buffers are all 16 bytes. (80 bytes before my earlier
> patches, 48 bytes after.)
>
> So the the structure goes from:
>
> 32-bit 64-bit Variable
> Offset Size Offset Size
> 0 2 0 2 spinlock_t prng_lock
> 2 16 2 16 unsigned char rand_data[16]
> 18 16 18 16 unsigned char DT[16]
> 34 16 34 16 unsigned char V[16]
> 50 2 50 2 (alignemnt)
> 52 4 52 4 u32 rand_read_pos
> 56 4 56 8 struct crypto_cipher *tfm
> 60 4 64 4 u32 flags
> 68 4 (alignment)
> 64 72 (structure size)
>
> to
>
> 32-bit 64-bit Variable
> Offset Size Offset Size
> 34 16 34 16 unsigned char V[16]
> 50 1 50 1 u8 rand_read_pos
> 51 1 51 1 u8 flags
> 52 4 (alignment)
> 52 4 56 8 struct crypto_cipher *tfm
> 56 64 (structure size)
>
> You still get 4 bytes of alignment padding on x86-64, but given that
> the structure has 60 bytes of payload, that's the minimum possible.
>
> You save 6 bytes of variables and 2 bytes of padding on both
> 32- and 64-bit systems, for a total of 8 bytes, and that's enough
> to knock you into a smaller slab object bin on 64-bit.
>
>
> I forget where I read the terminology, but the most efficient
> wat to pack a structure is in an "organ pipe" configuraiton where
> the biggest (in terms of *alignment*) members are on the outside
> and the structre and the smaller elements are on the inside.
> Putting a 32-bit "flags" after a 64-bit pointer violates that.
>
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