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

[Jim Mattson]

This might be more useful if it could be dynamically toggled on and
off, depending on system load.

On Tue, Apr 11, 2017 at 4:45 AM, Alexander Graf <address@hidden> wrote:
> From: "Michael S. Tsirkin" <address@hidden>
>
> 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.
>
> 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" <address@hidden>
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> [agraf: fix amd, change commit message]
> Signed-off-by: Alexander Graf <address@hidden>
>
> ---
>
> 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
>