Messages in this thread | | | Date | Mon, 15 Sep 2003 14:13:35 -0400 (EDT) | From | Bill Davidsen <> | Subject | Re: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata |
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On Mon, 15 Sep 2003, Mikael Pettersson wrote:
> On Mon, 15 Sep 2003 08:11:12 -0400 (EDT), Bill Davidsen <davidsen@tmr.com> wrote: > > On Mon, 15 Sep 2003, Alan Cox wrote: > > > That disable you talk about is bloat. It also trashes the performance of > > > PIV boxes. In fact I checked out of interest - the disable hack > > > currently being used is adding *over* 300 bytes to my kernel as its > > > inlined repeatedly. So its larger, and it ruins performance for all > > > processors. > > > > The code to disable prefetch on Athlon is 300 bytes and hurts your PIV? > > Really? I'll dig back through the code, but I recall it as adding or > > deleting an entry in a table to enable prefetch. If it's affecting PIV the > > code to use prefetch is seriously broken. > > Bill, look in include/asm-i386/processor.h: > > extern inline void prefetch(const void *x) > { > if (cpu_data[0].x86_vendor == X86_VENDOR_AMD) > return; /* Some athlons fault if the address is bad */ > alternative_input(ASM_NOP4, > "prefetchnta (%1)", > X86_FEATURE_XMM, > "r" (x)); > } > > A dynamic test at each occurrence. That's truly horrible. > (And I'll hack it out of _my_ kernels ASAP. Can't imagine > I missed that one.)
That's exactly the type of thing a MINIMAL_CODE flag could drop unless Athlon support was enabled. However, the test should be at the caller(s), to avoid a call/return in the first place.
To avoid really ugly source code it would probably be desirable to just define a do_prefetch_if_supported() macro, and put any and all future magic there. If the fix is used the test for AMD isn't needed, I would just like to avoid having the test or the fix at all on my Intel systems. And if I read correctly, that errata will be fixed in future Athlon CPUs, so a better test will be needed anyway.
The code is appropriate for a generic X86 kernel, but not for one built to one specific CPU. Everyone can be happy through the magin of Kconfig ;-)
-- bill davidsen <davidsen@tmr.com> CTO, TMR Associates, Inc Doing interesting things with little computers since 1979.
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