Re: RFC: Adding a hw.features[2] sysctl

On 14/01/2008, Nathan Lay <nslay_at_comcast.net> wrote:

> > "
> > CPU: Dual Core AMD Opteron(tm) Processor 280 (2411.12-MHz K8-class CPU)
> >   Origin = "AuthenticAMD"  Id = 0x20f12  Stepping = 2
> >   Features=0x178bfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,MMX,FXSR,SSE,SSE2,HTT>
> >   Features2=0x1<SSE3>
> >   AMD Features=0xe2500800<SYSCALL,NX,MMX+,FFXSR,LM,3DNow!+,3DNow!>
> >   AMD Features2=0x3<LAHF,CMP>
> >   Cores per package: 2
> > "
> >
> > ? Would you need four separate MIBs? Have four separate bitmasks in
> > one MIB, what order in? Is there XXX Features3, what would happen
> > then?
> >
> >
> > Igor
> >
> >
> Make one contiguous bitmask that include all of them.  That way the
> bitmask can sit in one sysctl MIB.
>
> For example:
>
> typedef uint8_t cpu_features_t[4*4]; /* 4 32 bit masks */
>
> #define CPU_HAS_FEATURE(mask,feature) ( (mask)[(feature) >> 3] &
> (1<<((feature) & 0x07)) )
>
> /* CPU features - reflect bit positions */
> #define CPU_I386_SSE    1
> #define CPU_I386_SSE2    2
> ...
> #define CPU_AMD64_ ...
> etc, etc...
>
> Something along these lines.  This of course removes the elegant OR'd
> flags and you'd have to do something like
>
> cpu_features_t mask;
>
> sysctl( ... ); /* Get mask */
> if ( CPU_HAS_FEATURE(mask, CPU_I386_SSE) || CPU_HAS_FEATURE(mask,
> CPU_I386_SSE2) ) ) { ... }
>
> These bit positions could be reclaimed for use by other
> architectures...the programmer's code or makefile would have to check
> that the code is being compiled on the intended architecture, otherwise
> you might have unexpected results (e.g. CPU_I386_SSE might mean
> something else).

There'd have to be a lot more sanity checks at runtime, on top of the
checks in the makefile. It's all getting very nasty on the userland,
just like I imagined. I doubt I will persuade you guys about the
portability and abstraction of an ioctl, so just go ahead and code it
as a MIB. If people want to write BSD-specific code to take advantage
of it in the future they can deal with it there and then...


Igor