Re: [PATCH v5 2/4] KVM: X86: Add paravirt remote TLB flush

[Konrad Rzeszutek Wilk]

On Mon, Nov 13, 2017 at 02:01:18AM -0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
> Remote flushing api's does a busy wait which is fine in bare-metal
> scenario. But with-in the guest, the vcpus might have been pre-empted
> or blocked. In this scenario, the initator vcpu would end up
> busy-waiting for a long amount of time.
> 
> This patch set implements para-virt flush tlbs making sure that it does 
> not wait for vcpus that are sleeping. And all the sleeping vcpus flush 
> the tlb on guest enter.
> 
> The best result is achieved when we're overcommiting the host by running 
> multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching 
> vCPUs which are not scheduled and avoid the wait on the main CPU.
> 
> Test on a Haswell i7 desktop 4 cores (2HT), so 8 pCPUs, running ebizzy 
> in one linux guest.
> 
> ebizzy -M 
>               vanilla    optimized     boost
>  8 vCPUs       10152       10083       -0.68% 
> 16 vCPUs        1224        4866       297.5% 
> 24 vCPUs        1109        3871       249%
> 32 vCPUs        1025        3375       229.3% 
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  Documentation/virtual/kvm/cpuid.txt  |  4 ++++
>  arch/x86/include/uapi/asm/kvm_para.h |  2 ++
>  arch/x86/kernel/kvm.c                | 42 +++++++++++++++++++++++++++++++++++-
>  3 files changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 117066a..9693fcc 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -60,6 +60,10 @@ KVM_FEATURE_PV_DEDICATED           ||     8 || guest checks this feature bit
>                                     ||       || mizations such as usage of
>                                     ||       || qspinlocks.
>  ------------------------------------------------------------------------------
> +KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest checks this feature bit
> +                                   ||       || before enabling paravirtualized
> +                                   ||       || tlb flush.
> +------------------------------------------------------------------------------
>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
>                                     ||       || per-cpu warps are expected in
>                                     ||       || kvmclock.
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 6d66556..e267d83 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -26,6 +26,7 @@
>  #define KVM_FEATURE_PV_EOI		6
>  #define KVM_FEATURE_PV_UNHALT		7
>  #define KVM_FEATURE_PV_DEDICATED	8
> +#define KVM_FEATURE_PV_TLB_FLUSH	9
>  
>  /* The last 8 bits are used to indicate how to interpret the flags field
>   * in pvclock structure. If no bits are set, all flags are ignored.
> @@ -54,6 +55,7 @@ struct kvm_steal_time {
>  
>  #define KVM_VCPU_NOT_PREEMPTED      (0 << 0)
>  #define KVM_VCPU_PREEMPTED          (1 << 0)
> +#define KVM_VCPU_SHOULD_FLUSH       (1 << 1)
>  
>  #define KVM_CLOCK_PAIRING_WALLCLOCK 0
>  struct kvm_clock_pairing {
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 66ed3bc..78794c1 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -465,9 +465,40 @@ static void __init kvm_apf_trap_init(void)
>  	update_intr_gate(X86_TRAP_PF, async_page_fault);
>  }
>  
> +static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
> +
> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
> +			const struct flush_tlb_info *info)

Something is off there..
> +{
> +	u8 state;
> +	int cpu;
> +	struct kvm_steal_time *src;
> +	struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
> +
> +	if (unlikely(!flushmask))
> +		return;
> +
> +	cpumask_copy(flushmask, cpumask);
> +	/*
> +	 * We have to call flush only on online vCPUs. And
> +	 * queue flush_on_enter for pre-empted vCPUs
> +	 */
> +	for_each_cpu(cpu, flushmask) {
> +		src = &per_cpu(steal_time, cpu);
> +		state = READ_ONCE(src->preempted);
> +		if ((state & KVM_VCPU_PREEMPTED)) {
> +			if (try_cmpxchg(&src->preempted, &state,
> +				state | KVM_VCPU_SHOULD_FLUSH))
> +				__cpumask_clear_cpu(cpu, flushmask);
> +		}
> +	}
> +
> +	native_flush_tlb_others(flushmask, info);
> +}
> +
>  void __init kvm_guest_init(void)
>  {
> -	int i;
> +	int i, cpu;
>  
>  	if (!kvm_para_available())
>  		return;
> @@ -484,6 +515,15 @@ void __init kvm_guest_init(void)
>  		pv_time_ops.steal_clock = kvm_steal_clock;
>  	}
>  
> +	if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
> +		!kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) {
> +		for_each_possible_cpu(cpu) {
> +			zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
> +					GFP_KERNEL, cpu_to_node(cpu));
> +		}
> +		pv_mmu_ops.flush_tlb_others = kvm_flush_tlb_others;

If we migrate to another host that does not expose this, should the
flush_tlb_others be reset back to the generic one? Or we don't care that
much ?

It seems to me that we would end up copying the cpumask, then loop around over
all the CPUs and then call native_flush_tlb_others again. Not sure if there
is much of performance problem there, but what is the historical way
this is handled?


> +	}
> +
>  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>  		apic_set_eoi_write(kvm_guest_apic_eoi_write);
>  
> -- 
> 2.7.4
>