Re: ZFS committed to the FreeBSD base.

On 2007-Apr-11 17:49:11 -0400, David Schultz <das@xxxxxxxxxxx> wrote:
As I recall, Solaris 10 targets PPro and later processors, whereas
FreeBSD supports everything back to a 486DX. Hence we can't
assume that cmpxchg8b is available.

There's a feature bit (CPUID_CX8) that advertises the availability of
cmpxchg8b (and maybe some related instructions). My pre-MMX 586 has
this bit set so I presume anything later than 486 will support it.
(I'm not sure about the low-end VIA, GEODE etc clones).

The last time I remember this
coming up, people argued that we had to do things slow way in the
default kernel for compatibility.

I agree that GENERIC should run on lowest-common-denominator hardware
(the definition of that is a subject for a different thread). GENERIC
performance could be enhanced by using an indirect call for 8-byte
atomic instructions and selecting between the cmpxchg8b and
alternative implementation as part of the CPU startup (much like
i586_bcopy). If CPU_486 is not defined, you code could inline the
cmpxchg8b-based variant.

--
Peter Jeremy

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