Re: [PATCH] KVM: VMX: Optimize tscdeadline timer latency

["Radim Krčmář" <rkrcmar@redhat.com>]

2018-05-29 14:53+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline 
> hrtimer expiration")' advances the tscdeadline (the timer is emulated 
> by hrtimer) expiration in order that the latency which is incurred 
> by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch 
> adds the advance tscdeadline expiration support to which the tscdeadline 
> timer is emulated by VMX preemption timer to reduce the hypervisor 
> lantency (handle_preemption_timer -> vmentry). clockevents infrastruture 
> can program minimum delay if hrtimer feeds a expiration in the past, 
> we set delta_tsc to 1(which will be converted to 0 before vmentry) 
> which can lead to an immediately vmexit when delta_tsc is not bigger 
> than advance ns. 
> 
> This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on 
> a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> busy waits.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
>  	tscl = rdtsc();
>  	guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
>  	delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> +	lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
> +	if (delta_tsc > lapic_timer_advance_cycles)
> +		delta_tsc -= lapic_timer_advance_cycles;
> +	else
> +		delta_tsc = 1;

Why don't we just "return 1" to say that the timer has expired?

I think "delta_tsc = 1" would just force an immediate VM exit and
a re-entry, which seems wasteful as we could just be delaying the entry
until the deadline has really passed,

thanks.