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

[Gabriel L. Somlo]

On Tue, Apr 11, 2017 at 08:41:11AM -0400, Gabriel L. Somlo wrote:
> On Tue, Apr 11, 2017 at 01:45:35PM +0200, Alexander Graf 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.
> 
> Did you mean "hlt" rather than "jmp" ?

Or, rather, "nop" ? (apologies, still waiting for the coffee to
cool down to a drinkable temperature... :)
 
> > 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>
> 
> 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 <address@hidden>
> 
> would be a more accurate statement instead?
> 
> Thanks much,
> --Gabriel
> 
> > 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
> >