Re: [RFC v2 05/83] Add NOVA filesystem definitions and useful helper routines.

Andiry Xu
2018-03-11T21:46:00Z

On Sun, Mar 11, 2018 at 12:22 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> On Sun, Mar 11, 2018 at 02:00:13PM +0200, Nikolay Borisov wrote:
>> [Adding Herbert Xu to CC since he is the maintainer of the crypto subsys
>> maintainer]
>>
>> On 10.03.2018 20:17, Andiry Xu wrote:
>> <snip>
>>
>> > +static inline u32 nova_crc32c(u32 crc, const u8 *data, size_t len)
>> > +{
>> > + u8 *ptr = (u8 *) data;
>> > + u64 acc = crc; /* accumulator, crc32c value in lower 32b */
>> > + u32 csum;
>> > +
>> > + /* x86 instruction crc32 is part of SSE-4.2 */
>> > + if (static_cpu_has(X86_FEATURE_XMM4_2)) {
>> > + /* This inline assembly implementation should be equivalent
>> > + * to the kernel's crc32c_intel_le_hw() function used by
>> > + * crc32c(), but this performs better on test machines.
>> > + */
>> > + while (len > 8) {
>> > + asm volatile(/* 64b quad words */
>> > + "crc32q (%1), %0"
>> > + : "=r" (acc)
>> > + : "r" (ptr), "0" (acc)
>> > + );
>> > + ptr += 8;
>> > + len -= 8;
>> > + }
>> > +
>> > + while (len > 0) {
>> > + asm volatile(/* trailing bytes */
>> > + "crc32b (%1), %0"
>> > + : "=r" (acc)
>> > + : "r" (ptr), "0" (acc)
>> > + );
>> > + ptr++;
>> > + len--;
>> > + }
>> > +
>> > + csum = (u32) acc;
>> > + } else {
>> > + /* The kernel's crc32c() function should also detect and use the
>> > + * crc32 instruction of SSE-4.2. But calling in to this function
>> > + * is about 3x to 5x slower than the inline assembly version on
>> > + * some test machines.
>>
>> That is really odd. Did you try to characterize why this is the case? Is
>> it purely the overhead of dispatching to the correct backend function?
>> That's a rather big performance hit.
>>
>> > + */
>> > + csum = crc32c(crc, data, len);
>> > + }
>> > +
>> > + return csum;
>> > +}
>> > +
>
> Are you sure that CONFIG_CRYPTO_CRC32C_INTEL was enabled during your tests and
> that the accelerated version was being called? Or, perhaps CRC32C_PCL_BREAKEVEN
> (defined in arch/x86/crypto/crc32c-intel_glue.c) needs to be adjusted. Please
> don't hack around performance problems like this; if they exist, they need to be
> fixed for everyone.
>

I think we found the issue when implementing NOVA-Fortis metadata and
data protections, which use crc32c a lot. They have been removed in
this patchset; but I will double check and make sure if the issue
exists or not.

Thanks,
Andiry

> Eric