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Date: Fri, 9 Mar 2018 19:12:33 +0100
From: Radim Krčmář <rkrcmar@...hat.com>
To: Babu Moger <babu.moger@....com>
Cc: joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
hpa@...or.com, x86@...nel.org, pbonzini@...hat.com,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/3] arch/x86/kvm: SVM: Introduce pause loop exit logic in
SVM
2018-03-02 11:17-0500, Babu Moger:
> Bring the PLE(pause loop exit) logic to AMD svm driver.
> We have noticed it help in situations where numerous pauses are generated
> due to spinlock or other scenarios. Tested it with idle=poll and noticed
> pause interceptions go down considerably.
>
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> arch/x86/kvm/svm.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/x86.h | 1 +
> 2 files changed, 114 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 50a4e95..30bc851 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -263,6 +263,55 @@ struct amd_svm_iommu_ir {
> static bool npt_enabled;
> #endif
>
> +/*
> + * These 2 parameters are used to config the controls for Pause-Loop Exiting:
> + * pause_filter_thresh: On processors that support Pause filtering(indicated
> + * by CPUID Fn8000_000A_EDX), the VMCB provides a 16 bit pause filter
> + * count value. On VMRUN this value is loaded into an internal counter.
> + * Each time a pause instruction is executed, this counter is decremented
> + * until it reaches zero at which time a #VMEXIT is generated if pause
> + * intercept is enabled. Refer to AMD APM Vol 2 Section 15.14.4 Pause
> + * Intercept Filtering for more details.
> + * This also indicate if ple logic enabled.
> + *
> + * pause_filter_count: In addition, some processor families support advanced
The comment has thresh/count flipped.
> + * pause filtering (indicated by CPUID Fn8000_000A_EDX) upper bound on
> + * the amount of time a guest is allowed to execute in a pause loop.
> + * In this mode, a 16-bit pause filter threshold field is added in the
> + * VMCB. The threshold value is a cycle count that is used to reset the
> + * pause counter. As with simple pause filtering, VMRUN loads the pause
> + * count value from VMCB into an internal counter. Then, on each pause
> + * instruction the hardware checks the elapsed number of cycles since
> + * the most recent pause instruction against the pause filter threshold.
> + * If the elapsed cycle count is greater than the pause filter threshold,
> + * then the internal pause count is reloaded from the VMCB and execution
> + * continues. If the elapsed cycle count is less than the pause filter
> + * threshold, then the internal pause count is decremented. If the count
> + * value is less than zero and PAUSE intercept is enabled, a #VMEXIT is
> + * triggered. If advanced pause filtering is supported and pause filter
> + * threshold field is set to zero, the filter will operate in the simpler,
> + * count only mode.
> + */
> +
> +static int pause_filter_thresh = KVM_DEFAULT_PLE_GAP;
> +module_param(pause_filter_thresh, int, S_IRUGO);
I think it was a mistake to put signed values in VMX ...
Please use unsigned variants and also properly sized.
(The module param type would be "ushort" instead of "int".)
> +static int pause_filter_count = KVM_DEFAULT_PLE_WINDOW;
> +module_param(pause_filter_count, int, S_IRUGO);
We are going to want a different default for pause_filter_count, because
they have a different meaning. On Intel, it's the number of cycles, on
AMD, it's the number of PAUSE instructions.
The AMD's 3k is a bit high in comparison to Intel's 4k, but I'd keep 3k
unless we have other benchmark results.
> +static int ple_window_grow = KVM_DEFAULT_PLE_WINDOW_GROW;
The naming would be nicer with a consistent prefix. We're growing
pause_filter_count, so pause_filter_count_grow is easier to understand.
(Albeit unwieldy.)
> +module_param(ple_window_grow, int, S_IRUGO);
(This is better as unsigned too ... VMX should have had that.)
> @@ -1046,6 +1095,58 @@ static int avic_ga_log_notifier(u32 ga_tag)
> return 0;
> }
>
> +static void grow_ple_window(struct kvm_vcpu *vcpu)
> +{
> + struct vcpu_svm *svm = to_svm(vcpu);
> + struct vmcb_control_area *control = &svm->vmcb->control;
> + int old = control->pause_filter_count;
> +
> + control->pause_filter_count = __grow_ple_window(old,
> + pause_filter_count,
> + ple_window_grow,
> + ple_window_actual_max);
> +
> + if (control->pause_filter_count != old)
> + mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
> +
> + trace_kvm_ple_window_grow(vcpu->vcpu_id,
> + control->pause_filter_count, old);
Please move the tracing into __shrink_ple_window to share the code.
This probably belongs to patch [2/3].
> +/*
> + * ple_window_actual_max is computed to be one grow_ple_window() below
> + * ple_window_max. (See __grow_ple_window for the reason.)
> + * This prevents overflows, because ple_window_max is int.
> + * ple_window_max effectively rounded down to a multiple of ple_window_grow in
> + * this process.
> + * ple_window_max is also prevented from setting control->pause_filter_count <
> + * pause_filter_count.
> + */
> +static void update_ple_window_actual_max(void)
> +{
> + ple_window_actual_max =
> + __shrink_ple_window(max(ple_window_max, pause_filter_count),
(I have no idea what I was thinking when I wrote that for VMX. :[
I'll write a patch to get rid of ple_window_actual_max, because its
benefits are really minuscule and the logic is complicated.)
> + pause_filter_count,
> + ple_window_grow, SHRT_MIN);
> +}
> static __init int svm_hardware_setup(void)
> {
> int cpu;
> @@ -1309,7 +1412,11 @@ static void init_vmcb(struct vcpu_svm *svm)
> svm->vcpu.arch.hflags = 0;
>
> if (boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
> - control->pause_filter_count = 3000;
> + control->pause_filter_count = pause_filter_count;
> + if (boot_cpu_has(X86_FEATURE_PFTHRESHOLD))
> + control->pause_filter_thresh = pause_filter_thresh;
> + else
> + pause_filter_thresh = 0;
Please move this to hardware_setup and also clear pause_filter_count if
X86_FEATURE_PAUSEFILTER is not present.
> set_intercept(svm, INTERCEPT_PAUSE);
The intercept should then be disabled iff pause_filter_count == 0.
The functionality looks correct,
thanks!
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