Improve detection of pvclock

There are two MSR variants, old-style and new-style. We want to prefer
the new-style one, and neither if the relevant flags aren't set in
CPUID.

diff --git a/platform/hw/arch/x86/clock.c b/platform/hw/arch/x86/clock.c
index 2d7e052ea13397fe8facf3b4133dd92196a7ad2e..4dbc8b00fad8e21f7e6439b369e8f737affafe7c 100644 (file)
--- a/platform/hw/arch/x86/clock.c
+++ b/platform/hw/arch/x86/clock.c
@@ -340,17 +340,35 @@ pvclock_read_wall_clock(void)
 
 /*
  * Initialise PV clock. Returns zero if successful (PV clock is available).
+ *
+ * Source: Linux kernel, Documentation/virtual/kvm/{msr,cpuid}.txt
  */
 static int
 pvclock_init(void)
 {
+       uint32_t eax, ebx, ecx, edx;
+       uint32_t msr_kvm_system_time, msr_kvm_wall_clock;
 
        if (hypervisor_detect() != HYPERVISOR_KVM)
                return 1;
+       /*
+        * Prefer new-style MSRs, and bail entirely if neither is indicated as
+        * available by CPUID.
+        */
+       x86_cpuid(0x40000001, &eax, &ebx, &ecx, &edx);
+       if (eax & (1 << 3)) {
+               msr_kvm_system_time = 0x4b564d01;
+               msr_kvm_wall_clock = 0x4b564d00;
+       }
+       else if (eax & (1 << 0)) {
+               msr_kvm_system_time = 0x12;
+               msr_kvm_wall_clock = 0x11;
+       }
+       else
+               return 1;
 
-       /* Initialise MSR_KVM_SYSTEM_TIME_NEW and enable updates */
        __asm__ __volatile("wrmsr" ::
-               "c" (0x4b564d01),
+               "c" (msr_kvm_system_time),
                "a" ((uint32_t)((uintptr_t)&pvclock_ti | 0x1)),
 #if defined(__x86_64__)
                "d" ((uint32_t)((uintptr_t)&pvclock_ti >> 32))
@@ -358,9 +376,8 @@ pvclock_init(void)
                "d" (0)
 #endif
        );
-       /* Initialise MSR_KVM_WALL_CLOCK_NEW */
        __asm__ __volatile("wrmsr" ::
-               "c" (0x4b564d00),
+               "c" (msr_kvm_wall_clock),
                "a" ((uint32_t)((uintptr_t)&pvclock_wc)),
 #if defined(__x86_64__)
                "d" ((uint32_t)((uintptr_t)&pvclock_wc >> 32))