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Date: Wed, 11 Jul 2018 18:28:51 +0200 From: Sebastian Andrzej Siewior <bigeasy@...utronix.de> To: "Jason A. Donenfeld" <Jason@...c4.com> Cc: Andrew Lutomirski <luto@...nel.org>, riel@...riel.com, dave.hansen@...ux.intel.com, LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org> Subject: Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch On 2018-06-15 22:33:47 [+0200], Jason A. Donenfeld wrote: > On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@...nel.org> wrote: > > quite in the form you imagined. The idea that we've tossed around is > > to restore FPU state on return to user mode. Roughly, we'd introduce > > a new thread flag TIF_FPU_UNLOADED (name TBD). > > prepare_exit_to_usermode() would notice this flag, copy the fpstate to > > fpregs, and clear the flag. (Or maybe exit_to_usermode_loop() -- No > > one has quite thought it through, but I think it should be outside the > > loop.) We'd update all the FPU accessors to understand the flag. > > Yes! This is exactly what I was thinking. Then those calls to begin() > and end() could be placed as close to the actual FPU usage as > possible. I was thinking about this myself. Did anyone try to hack something in the meantime? I might want to look into this, too :) Sebastian
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