This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: x86-vdso-pvclock-simplify-and-speed-up-the-vdso-pvclock-reader.patch and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 6b078f5de7fc0851af4102493c7b5bb07e49c4cb Mon Sep 17 00:00:00 2001

From: Andy Lutomirski luto@amacapital.net Date: Thu, 10 Dec 2015 19:20:19 -0800 Subject: x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

From: Andy Lutomirski luto@amacapital.net

commit 6b078f5de7fc0851af4102493c7b5bb07e49c4cb upstream.

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... Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Jamie Iles jamie.iles@oracle.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- arch/x86/entry/vdso/vclock_gettime.c | 79 +++++++++++++++++++---------------- 1 file changed, 45 insertions(+), 34 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c +++ b/arch/x86/entry/vdso/vclock_gettime.c @@ -78,47 +78,58 @@ static notrace const struct pvclock_vsys

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;

Patches currently in stable-queue which might be from luto@amacapital.net are

queue-4.4/x86-vdso-pvclock-simplify-and-speed-up-the-vdso-pvclock-reader.patch queue-4.4/x86-vdso-get-pvclock-data-from-the-vvar-vma-instead-of-the-fixmap.patch