[PATCH] x86: fix rdmsr MSR_PLATFORM_INFO unsafe warning in kvm guest

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at arch/x86/mm/extable.c:50 ex_handler_rdmsr_unsafe+0x6a/0x70
unchecked MSR access error: RDMSR from 0xce
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.7.0-rc3+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 0000000000000000 ffffffff81c03ce0 ffffffff813b3eae ffffffff81c03d30
 0000000000000000 ffffffff81c03d20 ffffffff81067181 0000003200000001
 ffffffff81c03df8 ffffffff8179676c 0000000000000000 ffffffff81fcd2c0
Call Trace:
 dump_stack+0x67/0x99
 __warn+0xd1/0xf0
 warn_slowpath_fmt+0x4f/0x60
 ex_handler_rdmsr_unsafe+0x6a/0x70
 fixup_exception+0x39/0x50
 do_general_protection+0x93/0x1b0
 general_protection+0x22/0x30
 ? cpu_khz_from_msr+0xd8/0x1c0
 native_calibrate_cpu+0x30/0x5b0
 tsc_init+0x2b/0x297
 x86_late_time_init+0xf/0x11
 start_kernel+0x398/0x451
 ? set_init_arg+0x55/0x55
 x86_64_start_reservations+0x2f/0x31
 x86_64_start_kernel+0xea/0xed

After commit (fc141535ad8 : "x86 tsc_msr: Extend to include Intel Core 
Architecture"), rdmsr MSR_PLATFORM_INFO is used to get maximum non-turbo 
ratio for recent Intel Core Architecture which results in kvm guest rdmsr 
unsafe warning.

As Radim pointed out before:

| MSR_PLATFORM_INFO: Intel changes it from family to family and there is
| no obvious overlap or default.  If we picked 0 (any other fixed value),
| then the guest would have to know that 0 doesn't mean that
| MSR_PLATFORM_INFO returned 0, but that KVM doesn't emulate this MSR and
| the value cannot be used.  This is very similar to handling a #GP in the
| guest, but also has a disadvantage, because KVM cannot say that
| MSR_PLATFORM_INFO is 0.  Simple emulation is not possible.

This patch fix it by using rdmsr_safe to read MSR_PLATFORM_INFO in kvm 
guest in order that #GP can be fixed up, then tsc will be calibrated by 
PIT, HPET etc.

Reported-by: kernel test robot <xiaolong.ye@intel.com>
Cc: Len Brown <len.brown@intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Chen Yu <y.c.chen@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: jacob.jun.pan@intel.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kernel/tsc_msr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index e0c2b30..e6e465e 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -70,7 +70,7 @@ static int match_cpu(u8 family, u8 model)
  */
 unsigned long cpu_khz_from_msr(void)
 {
-	u32 lo, hi, ratio, freq_id, freq;
+	u32 lo, hi, freq_id, freq, ratio = 0;
 	unsigned long res;
 	int cpu_index;
 
@@ -123,8 +123,8 @@ unsigned long cpu_khz_from_msr(void)
 	}
 
 get_ratio:
-	rdmsr(MSR_PLATFORM_INFO, lo, hi);
-	ratio = (lo >> 8) & 0xff;
+	if (!rdmsr_safe(MSR_PLATFORM_INFO, &lo, &hi))
+		ratio = (lo >> 8) & 0xff;
 
 done:
 	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
-- 
1.9.1