linux-kernel - [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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Message-Id: <b0ba174ea882ed36cf7011e872baf427c23b7e09.1424458621.git.luto@amacapital.net>
Date:	Fri, 20 Feb 2015 10:58:15 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>, Rik van Riel <riel@...hat.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>
Subject: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

We have eager and lazy fpu modes, introduced in:

304bceda6a18 x86, fpu: use non-lazy fpu restore for processors supporting xsave

The result is rather messy.  There are two code paths in almost all of the
FPU code, and only one of them (the eager case) is tested frequently, since
most kernel developers have new enough hardware that we use eagerfpu.

It seems that, on any remotely recent hardware, eagerfpu is a win:
glibc uses SSE2, so laziness is probably overoptimistic, and, in any
case, manipulating TS is far slower that saving and restoring the full
state.

To try to shake out any latent issues on old hardware, this changes
the default to eager on all CPUs.  If no performance or functionality
problems show up, a subsequent patch could remove lazy mode entirely.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/kernel/xsave.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 0de1fae2bdf0..1928c8f34ce5 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -637,8 +637,8 @@ static void __init xstate_enable_boot_cpu(void)
 	prepare_fx_sw_frame();
 	setup_init_fpu_buf();

-	/* Auto enable eagerfpu for xsaveopt */
-	if (cpu_has_xsaveopt && eagerfpu != DISABLE)
+	/* Auto enable eagerfpu for everyone */
+	if (eagerfpu != DISABLE)
 		eagerfpu = ENABLE;

 	if (pcntxt_mask & XSTATE_EAGER) {
-- 
2.1.0

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