Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 15 Jun 2018 11:53:39 -0700 | Subject | Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch |
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On Fri, Jun 15, 2018 at 11:50 AM Dave Hansen <dave.hansen@linux.intel.com> wrote: > > On 06/15/2018 11:31 AM, Andy Lutomirski wrote: > > for (thing) { > > kernel_fpu_begin(); > > encrypt(thing); > > kernel_fpu_end(); > > } > > Don't forget that the processor has optimizations for this, too. The > "modified optimization" will notice that between: > > kernel_fpu_end(); -> XRSTOR > and > kernel_fpu_start(); -> XSAVE(S|OPT) > > the processor has not modified the states. It'll skip doing any writes > of the state. Doing what Andy is describing is still way better than > letting the processor do it, but you should just know up front that this > may not be as much of a win as you would expect.
Even with the modified optimization, kernel_fpu_end() still needs to reload the state that was trashed by the kernel FPU use. If the kernel is using something like AVX512 state, then kernel_fpu_end() will transfer an enormous amount of data no matter how clever the CPU is. And I think I once measured XSAVEOPT taking a hundred cycles or so even when RFBM==0, so it's not exactly super fast.
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