Re: [tip:x86/asm] x86/kvm: On KVM re-enable (e.g. after suspend), update clocks

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/12/2015 09:16, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  677a73a9aa5433ea728200c26a7b3506d5eaa92b
> Gitweb:     http://git.kernel.org/tip/677a73a9aa5433ea728200c26a7b3506d5eaa92b
> Author:     Andy Lutomirski <luto@kernel.org>
> AuthorDate: Thu, 10 Dec 2015 19:20:18 -0800
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Fri, 11 Dec 2015 08:56:02 +0100
> 
> x86/kvm: On KVM re-enable (e.g. after suspend), update clocks
> 
> This gets rid of the "did TSC go backwards" logic and just
> updates all clocks.  It should work better (no more disabling of
> fast timing) and more reliably (all of the clocks are actually
> updated).
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> 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/861716d768a1da6d1fd257b7972f8df13baf7f85.1449702533.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/kvm/x86.c | 75 +++---------------------------------------------------
>  1 file changed, 3 insertions(+), 72 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 00462bd..6e32e87 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -123,8 +123,6 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
>  unsigned int __read_mostly lapic_timer_advance_ns = 0;
>  module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
>  
> -static bool __read_mostly backwards_tsc_observed = false;
> -
>  #define KVM_NR_SHARED_MSRS 16
>  
>  struct kvm_shared_msrs_global {
> @@ -1671,7 +1669,6 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  					&ka->master_cycle_now);
>  
>  	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
> -				&& !backwards_tsc_observed
>  				&& !ka->boot_vcpu_runs_old_kvmclock;
>  
>  	if (ka->use_master_clock)
> @@ -7366,88 +7363,22 @@ int kvm_arch_hardware_enable(void)
>  	struct kvm_vcpu *vcpu;
>  	int i;
>  	int ret;
> -	u64 local_tsc;
> -	u64 max_tsc = 0;
> -	bool stable, backwards_tsc = false;
>  
>  	kvm_shared_msr_cpu_online();
>  	ret = kvm_x86_ops->hardware_enable();
>  	if (ret != 0)
>  		return ret;
>  
> -	local_tsc = rdtsc();
> -	stable = !check_tsc_unstable();
>  	list_for_each_entry(kvm, &vm_list, vm_list) {
>  		kvm_for_each_vcpu(i, vcpu, kvm) {
> -			if (!stable && vcpu->cpu == smp_processor_id())
> +			if (vcpu->cpu == smp_processor_id()) {
>  				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> -			if (stable && vcpu->arch.last_host_tsc > local_tsc) {
> -				backwards_tsc = true;
> -				if (vcpu->arch.last_host_tsc > max_tsc)
> -					max_tsc = vcpu->arch.last_host_tsc;
> +				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE,
> +						 vcpu);
>  			}
>  		}
>  	}
>  
> -	/*
> -	 * Sometimes, even reliable TSCs go backwards.  This happens on
> -	 * platforms that reset TSC during suspend or hibernate actions, but
> -	 * maintain synchronization.  We must compensate.  Fortunately, we can
> -	 * detect that condition here, which happens early in CPU bringup,
> -	 * before any KVM threads can be running.  Unfortunately, we can't
> -	 * bring the TSCs fully up to date with real time, as we aren't yet far
> -	 * enough into CPU bringup that we know how much real time has actually
> -	 * elapsed; our helper function, get_kernel_ns() will be using boot
> -	 * variables that haven't been updated yet.
> -	 *
> -	 * So we simply find the maximum observed TSC above, then record the
> -	 * adjustment to TSC in each VCPU.  When the VCPU later gets loaded,
> -	 * the adjustment will be applied.  Note that we accumulate
> -	 * adjustments, in case multiple suspend cycles happen before some VCPU
> -	 * gets a chance to run again.  In the event that no KVM threads get a
> -	 * chance to run, we will miss the entire elapsed period, as we'll have
> -	 * reset last_host_tsc, so VCPUs will not have the TSC adjusted and may
> -	 * loose cycle time.  This isn't too big a deal, since the loss will be
> -	 * uniform across all VCPUs (not to mention the scenario is extremely
> -	 * unlikely). It is possible that a second hibernate recovery happens
> -	 * much faster than a first, causing the observed TSC here to be
> -	 * smaller; this would require additional padding adjustment, which is
> -	 * why we set last_host_tsc to the local tsc observed here.
> -	 *
> -	 * N.B. - this code below runs only on platforms with reliable TSC,
> -	 * as that is the only way backwards_tsc is set above.  Also note
> -	 * that this runs for ALL vcpus, which is not a bug; all VCPUs should
> -	 * have the same delta_cyc adjustment applied if backwards_tsc
> -	 * is detected.  Note further, this adjustment is only done once,
> -	 * as we reset last_host_tsc on all VCPUs to stop this from being
> -	 * called multiple times (one for each physical CPU bringup).
> -	 *
> -	 * Platforms with unreliable TSCs don't have to deal with this, they
> -	 * will be compensated by the logic in vcpu_load, which sets the TSC to
> -	 * catchup mode.  This will catchup all VCPUs to real time, but cannot
> -	 * guarantee that they stay in perfect synchronization.
> -	 */
> -	if (backwards_tsc) {
> -		u64 delta_cyc = max_tsc - local_tsc;
> -		backwards_tsc_observed = true;
> -		list_for_each_entry(kvm, &vm_list, vm_list) {
> -			kvm_for_each_vcpu(i, vcpu, kvm) {
> -				vcpu->arch.tsc_offset_adjustment += delta_cyc;
> -				vcpu->arch.last_host_tsc = local_tsc;
> -				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
> -			}
> -
> -			/*
> -			 * We have to disable TSC offset matching.. if you were
> -			 * booting a VM while issuing an S4 host suspend....
> -			 * you may have some problem.  Solving this issue is
> -			 * left as an exercise to the reader.
> -			 */
> -			kvm->arch.last_tsc_nsec = 0;
> -			kvm->arch.last_tsc_write = 0;
> -		}
> -
> -	}
>  	return 0;
>  }
>  
> 

NACK.

Please remove this patch from the tip tree.

Paolo