[tip:x86/platform] x86/hyperv: Mark the Hyper-V TSC as unstable

[tip-bot for Vitaly Kuznetsov <tipbot@zytor.com>]

Commit-ID:  88c9281a9fba67636ab26c1fd6afbc78a632374f
Gitweb:     http://git.kernel.org/tip/88c9281a9fba67636ab26c1fd6afbc78a632374f
Author:     Vitaly Kuznetsov <vkuznets@redhat.com>
AuthorDate: Wed, 19 Aug 2015 09:54:24 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 21 Aug 2015 08:44:38 +0200

x86/hyperv: Mark the Hyper-V TSC as unstable

The Hyper-V top-level functional specification states, that
"algorithms should be resilient to sudden jumps forward or
backward in the TSC value", this means that we should consider
TSC as unstable. In some cases tsc tests are able to detect the
instability, it was detected in 543 out of 646 boots in my
testing:

 Measured 6277 cycles TSC warp between CPUs, turning off TSC clock.
 tsc: Marking TSC unstable due to check_tsc_sync_source failed

This is, however, just a heuristic. On Hyper-V platform there
are two good clocksources: MSR-based hyperv_clocksource and
recently introduced TSC page.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: devel@linuxdriverproject.org
Link: http://lkml.kernel.org/r/1440003264-9949-1-git-send-email-vkuznets@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/cpu/mshyperv.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index aad4bd8..6fd023d 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -141,6 +141,7 @@ static void __init ms_hyperv_init_platform(void)
 	no_timer_check = 1;
 #endif
 
+	mark_tsc_unstable("running on Hyper-V");
 }
 
 const __refconst struct hypervisor_x86 x86_hyper_ms_hyperv = {