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 <ag...@suse.de> wrote: > From: "Michael S. Tsirkin" <m...@redhat.com> > > 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" <gso...@gmail.com> > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > [agraf: fix amd, change commit message] > Signed-off-by: Alexander Graf <ag...@suse.de> > > --- > > 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 >