Re: [PATCH v6] kvm: better MWAIT emulation for guests

From: Gabriel L. Somlo
Date: Tue Apr 11 2017 - 08:41:27 EST

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On Tue, Apr 11, 2017 at 01:45:35PM +0200, Alexander Graf wrote:
> From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
>
> Guests that are heavy on futexes end up IPI'ing each other a lot. That
> can lead to significant slowdowns and latency increase for those guests
> when running within KVM.
>
> If only a single guest is needed on a host, we have a lot of spare host
> CPU time we can throw at the problem. Modern CPUs implement a feature
> called "MWAIT" which allows guests to wake up sleeping remote CPUs without
> an IPI - thus without an exit - at the expense of never going out of guest
> context.
>
> The decision whether this is something sensible to use should be up to the
> VM admin, so to user space. We can however allow MWAIT execution on systems
> that support it properly hardware wise.
>
> This patch adds a CAP to user space and a KVM cpuid leaf to indicate
> availability of native MWAIT execution. With that enabled, the worst a
> guest can do is waste as many cycles as a "jmp ." would do, so it's not
> a privilege problem.

Did you mean "hlt" rather than "jmp" ?

> We consciously do *not* expose the feature in our CPUID bitmap, as most
> people will want to benefit from sleeping vCPUs to allow for over commit.
>
> Reported-by: "Gabriel L. Somlo" <gsomlo@xxxxxxxxx>

That's maybe a bit inacurate, I didn't actually report anything *this*
patch is trying to address (that was rather commit 87c00572ba05aa8c9d).

Maybe

Acked-by: Gabriel Somlo <gsomlo@xxxxxxxxx>

would be a more accurate statement instead?

Thanks much,
--Gabriel

> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> [agraf: fix amd, change commit message]
> Signed-off-by: Alexander Graf <agraf@xxxxxxx>
>
> ---
>
> v5 -> v6:
>
> - Fix AMD check, so that we're consistent between svm and vmx
> - Clarify commit message
> ---
> Documentation/virtual/kvm/api.txt | 9 +++++++++
> Documentation/virtual/kvm/cpuid.txt | 6 ++++++
> arch/x86/include/uapi/asm/kvm_para.h | 1 +
> arch/x86/kvm/cpuid.c | 3 +++
> arch/x86/kvm/svm.c | 7 +++++--
> arch/x86/kvm/vmx.c | 6 ++++--
> arch/x86/kvm/x86.c | 3 +++
> arch/x86/kvm/x86.h | 36 ++++++++++++++++++++++++++++++++++++
> include/uapi/linux/kvm.h | 1 +
> 9 files changed, 68 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 1a18484..dacc3d7 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4094,3 +4094,12 @@ reserved.
> 2: MIPS64 or microMIPS64 with access to all address segments.
> Both registers and addresses are 64-bits wide.
> It will be possible to run 64-bit or 32-bit guest code.
> +
> +8.8 KVM_CAP_X86_GUEST_MWAIT
> +
> +Architectures: x86
> +
> +This capability indicates that guest using memory monotoring instructions
> +(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit. As such time
> +spent while virtual CPU is halted in this way will then be accounted for as
> +guest running time on the host (as opposed to e.g. HLT).
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 3c65feb..04c201c 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -54,6 +54,12 @@ KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit
> || || before enabling paravirtualized
> || || spinlock support.
> ------------------------------------------------------------------------------
> +KVM_FEATURE_MWAIT || 8 || guest can use monitor/mwait
> + || || to halt the VCPU without exits,
> + || || time spent while halted in this
> + || || way is accounted for on host as
> + || || VCPU run time.
> +------------------------------------------------------------------------------
> 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 cff0bb6..9cc77a7 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -24,6 +24,7 @@
> #define KVM_FEATURE_STEAL_TIME 5
> #define KVM_FEATURE_PV_EOI 6
> #define KVM_FEATURE_PV_UNHALT 7
> +#define KVM_FEATURE_MWAIT 8
>
> /* 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.
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index efde6cc..5638102 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -594,6 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> if (sched_info_on())
> entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
>
> + if (kvm_mwait_in_guest())
> + entry->eax |= (1 << KVM_FEATURE_MWAIT);
> +
> entry->ebx = 0;
> entry->ecx = 0;
> entry->edx = 0;
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 1b203ab..c41f03e 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1198,10 +1198,13 @@ static void init_vmcb(struct vcpu_svm *svm)
> set_intercept(svm, INTERCEPT_CLGI);
> set_intercept(svm, INTERCEPT_SKINIT);
> set_intercept(svm, INTERCEPT_WBINVD);
> - set_intercept(svm, INTERCEPT_MONITOR);
> - set_intercept(svm, INTERCEPT_MWAIT);
> set_intercept(svm, INTERCEPT_XSETBV);
>
> + if (!kvm_mwait_in_guest()) {
> + set_intercept(svm, INTERCEPT_MONITOR);
> + set_intercept(svm, INTERCEPT_MWAIT);
> + }
> +
> control->iopm_base_pa = iopm_base;
> control->msrpm_base_pa = __pa(svm->msrpm);
> control->int_ctl = V_INTR_MASKING_MASK;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index cfdb0d9..2e62f9d 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3547,11 +3547,13 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
> CPU_BASED_USE_IO_BITMAPS |
> CPU_BASED_MOV_DR_EXITING |
> CPU_BASED_USE_TSC_OFFSETING |
> - CPU_BASED_MWAIT_EXITING |
> - CPU_BASED_MONITOR_EXITING |
> CPU_BASED_INVLPG_EXITING |
> CPU_BASED_RDPMC_EXITING;
>
> + if (!kvm_mwait_in_guest())
> + min |= CPU_BASED_MWAIT_EXITING |
> + CPU_BASED_MONITOR_EXITING;
> +
> opt = CPU_BASED_TPR_SHADOW |
> CPU_BASED_USE_MSR_BITMAPS |
> CPU_BASED_ACTIVATE_SECONDARY_CONTROLS;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6bc47e2..ffaa76d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2679,6 +2679,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_ADJUST_CLOCK:
> r = KVM_CLOCK_TSC_STABLE;
> break;
> + case KVM_CAP_X86_GUEST_MWAIT:
> + r = kvm_mwait_in_guest();
> + break;
> case KVM_CAP_X86_SMM:
> /* SMBASE is usually relocated above 1M on modern chipsets,
> * and SMM handlers might indeed rely on 4G segment limits,
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index e8ff3e4..6120670 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -1,6 +1,8 @@
> #ifndef ARCH_X86_KVM_X86_H
> #define ARCH_X86_KVM_X86_H
>
> +#include <asm/processor.h>
> +#include <asm/mwait.h>
> #include <linux/kvm_host.h>
> #include <asm/pvclock.h>
> #include "kvm_cache_regs.h"
> @@ -212,4 +214,38 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
> __rem; \
> })
>
> +static inline bool kvm_mwait_in_guest(void)
> +{
> + unsigned int eax, ebx, ecx, edx;
> +
> + if (!cpu_has(&boot_cpu_data, X86_FEATURE_MWAIT))
> + return false;
> +
> + switch (boot_cpu_data.x86_vendor) {
> + case X86_VENDOR_AMD:
> + /* All AMD CPUs have a working MWAIT implementation */
> + return true;
> + case X86_VENDOR_INTEL:
> + /* Handle Intel below */
> + break;
> + default:
> + return false;
> + }
> +
> + /*
> + * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as
> + * they would allow guest to stop the CPU completely by disabling
> + * interrupts then invoking MWAIT.
> + */
> + if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
> + return false;
> +
> + cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &edx);
> +
> + if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK))
> + return false;
> +
> + return true;
> +}
> +
> #endif
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 1e1a6c7..a3f03d3 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -890,6 +890,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_MIPS_VZ 137
> #define KVM_CAP_MIPS_TE 138
> #define KVM_CAP_MIPS_64BIT 139
> +#define KVM_CAP_X86_GUEST_MWAIT 140
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 1.8.5.6
>

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In reply to: Alexander Graf: "[PATCH v6] kvm: better MWAIT emulation for guests"
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