x86: vdso: pvclock gettime support

Improve performance of time system calls when using Linux pvclock,
by reading time info from fixmap visible copy of pvclock data.

Originally from Jeremy Fitzhardinge.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

3 files changed, 101 insertions, 11 deletions

diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index eaea1d31f753..80f80955cfd8 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -33,6 +33,26 @@ extern void map_vsyscall(void);
  */
 extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
 
+#ifdef CONFIG_X86_64
+
+#define VGETCPU_CPU_MASK 0xfff
+
+static inline unsigned int __getcpu(void)
+{
+        unsigned int p;
+
+        if (VVAR(vgetcpu_mode) == VGETCPU_RDTSCP) {
+                /* Load per CPU data from RDTSCP */
+                native_read_tscp(&p);
+        } else {
+                /* Load per CPU data from GDT */
+                asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
+        }
+
+        return p;
+}
+#endif /* CONFIG_X86_64 */
+
 #endif /* __KERNEL__ */
 
 #endif /* _ASM_X86_VSYSCALL_H */
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 4df6c373421a..205ad328aa52 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -22,6 +22,7 @@
 #include <asm/hpet.h>
 #include <asm/unistd.h>
 #include <asm/io.h>
+#include <asm/pvclock.h>
 
 #define gtod (&VVAR(vsyscall_gtod_data))
 
@@ -62,6 +63,76 @@ static notrace cycle_t vread_hpet(void)
         return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
 }
 
+#ifdef CONFIG_PARAVIRT_CLOCK
+
+static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
+{
+        const struct pvclock_vsyscall_time_info *pvti_base;
+        int idx = cpu / (PAGE_SIZE/PVTI_SIZE);
+        int offset = cpu % (PAGE_SIZE/PVTI_SIZE);
+
+        BUG_ON(PVCLOCK_FIXMAP_BEGIN + idx > PVCLOCK_FIXMAP_END);
+
+        pvti_base = (struct pvclock_vsyscall_time_info *)
+                    __fix_to_virt(PVCLOCK_FIXMAP_BEGIN+idx);
+
+        return &pvti_base[offset];
+}
+
+static notrace cycle_t vread_pvclock(int *mode)
+{
+        const struct pvclock_vsyscall_time_info *pvti;
+        cycle_t ret;
+        u64 last;
+        u32 version;
+        u32 migrate_count;
+        u8 flags;
+        unsigned cpu, cpu1;
+
+
+        /*
+         * When looping to get a consistent (time-info, tsc) pair, we
+         * also need to deal with the possibility we can switch vcpus,
+         * so make sure we always re-fetch time-info for the current vcpu.
+         */
+        do {
+                cpu = __getcpu() & VGETCPU_CPU_MASK;
+                /* TODO: We can put vcpu id into higher bits of pvti.version.
+                 * This will save a couple of cycles by getting rid of
+                 * __getcpu() calls (Gleb).
+                 */
+
+                pvti = get_pvti(cpu);
+
+                migrate_count = pvti->migrate_count;
+
+                version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
+
+                /*
+                 * Test we're still on the cpu as well as the version.
+                 * We could have been migrated just after the first
+                 * vgetcpu but before fetching the version, so we
+                 * wouldn't notice a version change.
+                 */
+                cpu1 = __getcpu() & VGETCPU_CPU_MASK;
+        } while (unlikely(cpu != cpu1 ||
+                          (pvti->pvti.version & 1) ||
+                          pvti->pvti.version != version ||
+                          pvti->migrate_count != migrate_count));
+
+        if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
+                *mode = VCLOCK_NONE;
+
+        /* refer to tsc.c read_tsc() comment for rationale */
+        last = VVAR(vsyscall_gtod_data).clock.cycle_last;
+
+        if (likely(ret >= last))
+                return ret;
+
+        return last;
+}
+#endif
+
 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 {
         long ret;
@@ -80,7 +151,7 @@ notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
 }
 
 
-notrace static inline u64 vgetsns(void)
+notrace static inline u64 vgetsns(int *mode)
 {
         long v;
         cycles_t cycles;
@@ -88,6 +159,10 @@ notrace static inline u64 vgetsns(void)
                 cycles = vread_tsc();
         else if (gtod->clock.vclock_mode == VCLOCK_HPET)
                 cycles = vread_hpet();
+#ifdef CONFIG_PARAVIRT_CLOCK
+        else if (gtod->clock.vclock_mode == VCLOCK_PVCLOCK)
+                cycles = vread_pvclock(mode);
+#endif
         else
                 return 0;
         v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
@@ -107,7 +182,7 @@ notrace static int __always_inline do_realtime(struct timespec *ts)
                 mode = gtod->clock.vclock_mode;
                 ts->tv_sec = gtod->wall_time_sec;
                 ns = gtod->wall_time_snsec;
-                ns += vgetsns();
+                ns += vgetsns(&mode);
                 ns >>= gtod->clock.shift;
         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 
@@ -127,7 +202,7 @@ notrace static int do_monotonic(struct timespec *ts)
                 mode = gtod->clock.vclock_mode;
                 ts->tv_sec = gtod->monotonic_time_sec;
                 ns = gtod->monotonic_time_snsec;
-                ns += vgetsns();
+                ns += vgetsns(&mode);
                 ns >>= gtod->clock.shift;
         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
         timespec_add_ns(ts, ns);
diff --git a/arch/x86/vdso/vgetcpu.c b/arch/x86/vdso/vgetcpu.c
index 5463ad558573..2f94b039e55b 100644
--- a/arch/x86/vdso/vgetcpu.c
+++ b/arch/x86/vdso/vgetcpu.c
@@ -17,15 +17,10 @@ __vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
 {
         unsigned int p;
 
-        if (VVAR(vgetcpu_mode) == VGETCPU_RDTSCP) {
-                /* Load per CPU data from RDTSCP */
-                native_read_tscp(&p);
-        } else {
-                /* Load per CPU data from GDT */
-                asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
-        }
+        p = __getcpu();
+
         if (cpu)
-                *cpu = p & 0xfff;
+                *cpu = p & VGETCPU_CPU_MASK;
         if (node)
                 *node = p >> 12;
         return 0;