Messages in this thread | | | Date | Tue, 11 Mar 2008 13:19:22 -0700 | From | Suresh Siddha <> | Subject | Re: [patch 1/2] x86, fpu: split FPU state from task struct - v5 |
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On Tue, Mar 11, 2008 at 08:07:34AM +0300, Alexey Dobriyan wrote: > On Mon, Mar 10, 2008 at 03:28:04PM -0700, Suresh Siddha wrote: > > Split the FPU save area from the task struct. This allows easy migration > > of FPU context, and it's generally cleaner. It also allows the following > > two optimizations: > > > > 1) only allocate when the application actually uses FPU, so in the first > > lazy FPU trap. This could save memory for non-fpu using apps. Next patch > > does this lazy allocation. > > > > 2) allocate the right size for the actual cpu rather than 512 bytes always. > > Patches enabling xsave/xrstor support (coming shortly) will take advantage > > of this. > > Ugh, not seeing patch, but judging from description it will make > "choose wrong CONFIG_M* and fxsave will corrupt random FPU state" situation > likely?
No. CONFIG_M* doesn't determine the size of the state. Feature information from the 'cpuid' instruction will dictate the size allocated/used. Anyhow, please wait for the xsave patches.
> > > --- linux-2.6-x86.orig/arch/x86/kernel/process_64.c > > +++ linux-2.6-x86/arch/x86/kernel/process_64.c > > @@ -634,7 +634,7 @@ > > > > /* we're going to use this soon, after a few expensive things */ > > if (next_p->fpu_counter>5) > > - prefetch(&next->i387.fxsave); > > + prefetch(next->xstate); > > Can we please give it better name, like fpu_state? It's a member of > task_struct after all.
It need not be only FPU. We can have non-math state here aswell.
selected 'xstate' for extended state. I am all open for any reasonable name, reflecting math, extended math(fsave/fxsave/..) and future math/ non-math extensions.
> > { > > unsigned long oldcr0 = read_cr0(); > > - extern void __bad_fxsave_alignment(void); > > - > > - if (offsetof(struct task_struct, thread.i387.fxsave) & 15) > > - __bad_fxsave_alignment(); > > I think removal of such checks needs giving necessary alignment to cache. > Previously it worked because of __aligned((16)) and L1_CACHE_SHIFT > combo.
alignment is now specified as part of kmem_cache_create() and checed in the allocation routines.
thanks, suresh
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