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x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader 

The pvclock vdso code was too abstracted to understand easily
and excessively paranoid.  Simplify it for a huge speedup.

This opens the door for additional simplifications, as the vdso
no longer accesses the pvti for any vcpu other than vcpu 0.

Before, vclock_gettime using kvm-clock took about 45ns on my
machine. With this change, it takes 29ns, which is almost as
fast as the pure TSC implementation.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/6b51dcc41f1b101f963945c5ec7093d72bdac429.1449702533.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2015-12-10 19:20:19 -08:00 committed by Ingo Molnar
parent 677a73a9aa
commit 6b078f5de7
1 changed files with 45 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

79
arch/x86/entry/vdso/vclock_gettime.c
View File

@ -78,47 +78,58 @@ static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
static notrace cycle_t vread_pvclock(int *mode)
{
const struct pvclock_vsyscall_time_info *pvti;
const struct pvclock_vcpu_time_info *pvti = &get_pvti(0)->pvti;
cycle_t ret;
u64 last;
u32 version;
u8 flags;
unsigned cpu, cpu1;
u64 tsc, pvti_tsc;
u64 last, delta, pvti_system_time;
u32 version, pvti_tsc_to_system_mul, pvti_tsc_shift;
/*
* Note: hypervisor must guarantee that:
* 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
* 2. that per-CPU pvclock time info is updated if the
* underlying CPU changes.
* 3. that version is increased whenever underlying CPU
* changes.
* Note: The kernel and hypervisor must guarantee that cpu ID
* number maps 1:1 to per-CPU pvclock time info.
*
* Because the hypervisor is entirely unaware of guest userspace
* preemption, it cannot guarantee that per-CPU pvclock time
* info is updated if the underlying CPU changes or that that
* version is increased whenever underlying CPU changes.
*
* On KVM, we are guaranteed that pvti updates for any vCPU are
* atomic as seen by *all* vCPUs. This is an even stronger
* guarantee than we get with a normal seqlock.
*
* On Xen, we don't appear to have that guarantee, but Xen still
* supplies a valid seqlock using the version field.
* We only do pvclock vdso timing at all if
* PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
* mean that all vCPUs have matching pvti and that the TSC is
* synced, so we can just look at vCPU 0's pvti.
*/
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);
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));
if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
*mode = VCLOCK_NONE;
return 0;
}
do {
version = pvti->version;
/* This is also a read barrier, so we'll read version first. */
tsc = rdtsc_ordered();
pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
pvti_tsc_shift = pvti->tsc_shift;
pvti_system_time = pvti->system_time;
pvti_tsc = pvti->tsc_timestamp;
/* Make sure that the version double-check is last. */
smp_rmb();
} while (unlikely((version & 1) || version != pvti->version));
delta = tsc - pvti_tsc;
ret = pvti_system_time +
pvclock_scale_delta(delta, pvti_tsc_to_system_mul,
pvti_tsc_shift);
/* refer to tsc.c read_tsc() comment for rationale */
last = gtod->cycle_last;