linux-kernel - [PATCH] x86/timers: Move simple_udelay_calibration() past kvmclock

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Date:   Mon, 11 Sep 2017 11:04:19 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     hpa@...or.com, mingo@...hat.com, tglx@...utronix.de
Cc:     baolu.lu@...ux.intel.com, linux-kernel@...r.kernel.org,
        x86@...nel.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: [PATCH] x86/timers: Move simple_udelay_calibration() past kvmclock_init()

simple_udelay_calibration() relies on x86_platform's calibration ops.
For KVM these ops are set late in setup_arch() and so
simple_udelay_calibration() ends up using native version.

Besides being possibly incorrect, this significantly increases kernel
boot time. For example, on my laptop executing start_kernel() by a guest
takes ~10 times more than when KVM's ops are used.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
---
Alternatively, we could start calling simple_udelay_calibration() only
on bare metal: My understanding is that the only reason it exists is to
help with USB3 earlyprink driver, which I don't think is useful to guests
anyway.


 arch/x86/kernel/setup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index f81823695014..749439fba076 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1039,8 +1039,6 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	init_hypervisor_platform();
 
-	simple_udelay_calibration();
-
 	x86_init.resources.probe_roms();
 
 	/* after parse_early_param, so could debug it */
@@ -1221,6 +1219,8 @@ void __init setup_arch(char **cmdline_p)
 	kvmclock_init();
 #endif
 
+	simple_udelay_calibration();
+
 	x86_init.paging.pagetable_init();
 
 	kasan_init();
-- 
2.11.0