Re: [PATCH v2 1/3] KVM: X86: Provides userspace with a capability to not intercept MWAIT

From: Jim Mattson
Date: Tue Mar 13 2018 - 14:21:32 EST

Is there a need for a new API for yielding MONITOR/MWAIT to the guest?
Why not just tie this to the guest CPUID.01H:ECX[MWAIT] being set?

On Mon, Mar 12, 2018 at 4:53 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> Allowing a guest to execute MWAIT without interception enables a guest
> to put a (physical) CPU into a power saving state, where it takes
> longer to return from than what may be desired by the host.
>
> Don't give a guest that power over a host by default. (Especially,
> since nothing prevents a guest from using MWAIT even when it is not
> advertised via CPUID.)
>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Jan H. SchÃnherr <jschoenh@xxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> Documentation/virtual/kvm/api.txt | 23 ++++++++++++++---------
> arch/x86/include/asm/kvm_host.h | 2 ++
> arch/x86/kvm/svm.c | 2 +-
> arch/x86/kvm/vmx.c | 9 +++++----
> arch/x86/kvm/x86.c | 24 ++++++++++++++++++++----
> arch/x86/kvm/x86.h | 10 +++++-----
> include/uapi/linux/kvm.h | 2 +-
> tools/include/uapi/linux/kvm.h | 2 +-
> 8 files changed, 49 insertions(+), 25 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 98de506..76e5a15 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4358,6 +4358,20 @@ enables QEMU to build error log and branch to guest kernel registered
> machine check handling routine. Without this capability KVM will
> branch to guests' 0x200 interrupt vector.
>
> +7.13 KVM_CAP_X86_DISABLE_EXITS
> +
> +Architectures: x86
> +Parameters: args[0] defines which exits are disabled
> +Returns: 0 on success, -EINVAL when args[0] contains invalid exits
> +
> +Valid exits in args[0] are
> +
> +#define KVM_X86_DISABLE_EXITS_MWAIT (1 << 0)
> +
> +Enabling this capability on a VM provides userspace with a way to no
> +longer intercepts some instructions for improved latency in some
> +workloads.
> +
> 8. Other capabilities.
> ----------------------
>
> @@ -4470,15 +4484,6 @@ reserved.
> 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).
> -
> 8.9 KVM_CAP_ARM_USER_IRQ
>
> Architectures: arm, arm64
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0395c35..e107171 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -811,6 +811,8 @@ struct kvm_arch {
>
> gpa_t wall_clock;
>
> + bool mwait_in_guest;
> +
> bool ept_identity_pagetable_done;
> gpa_t ept_identity_map_addr;
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index be9c839..321b3fd 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1390,7 +1390,7 @@ static void init_vmcb(struct vcpu_svm *svm)
> set_intercept(svm, INTERCEPT_XSETBV);
> set_intercept(svm, INTERCEPT_RSM);
>
> - if (!kvm_mwait_in_guest()) {
> + if (!kvm_mwait_in_guest(svm->vcpu.kvm)) {
> set_intercept(svm, INTERCEPT_MONITOR);
> set_intercept(svm, INTERCEPT_MWAIT);
> }
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 6cefd7b..2302ae2 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3733,13 +3733,11 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
> CPU_BASED_UNCOND_IO_EXITING |
> 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;
> @@ -5531,6 +5529,9 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
> exec_control |= CPU_BASED_CR3_STORE_EXITING |
> CPU_BASED_CR3_LOAD_EXITING |
> CPU_BASED_INVLPG_EXITING;
> + if (kvm_mwait_in_guest(vmx->vcpu.kvm))
> + exec_control &= ~(CPU_BASED_MWAIT_EXITING |
> + CPU_BASED_MONITOR_EXITING);
> return exec_control;
> }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 36ef3d8..5fae476 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2809,9 +2809,15 @@ static int msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs __user *user_msrs,
> return r;
> }
>
> +static inline bool kvm_can_mwait_in_guest(void)
> +{
> + return boot_cpu_has(X86_FEATURE_MWAIT) &&
> + !boot_cpu_has_bug(X86_BUG_MONITOR);
> +}
> +
> int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> {
> - int r;
> + int r = 0;
>
> switch (ext) {
> case KVM_CAP_IRQCHIP:
> @@ -2867,8 +2873,9 @@ 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();
> + case KVM_CAP_X86_DISABLE_EXITS:
> + if(kvm_can_mwait_in_guest())
> + r |= KVM_X86_DISABLE_EXITS_MWAIT;
> break;
> case KVM_CAP_X86_SMM:
> /* SMBASE is usually relocated above 1M on modern chipsets,
> @@ -2909,7 +2916,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> r = KVM_X2APIC_API_VALID_FLAGS;
> break;
> default:
> - r = 0;
> break;
> }
> return r;
> @@ -4214,6 +4220,16 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>
> r = 0;
> break;
> + case KVM_CAP_X86_DISABLE_EXITS:
> + r = -EINVAL;
> + if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
> + break;
> +
> + if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
> + kvm_can_mwait_in_guest())
> + kvm->arch.mwait_in_guest = true;
> + r = 0;
> + break;
> default:
> r = -EINVAL;
> break;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index b91215d..cd1215e 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -2,8 +2,6 @@
> #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"
> @@ -264,10 +262,12 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
> __rem; \
> })
>
> -static inline bool kvm_mwait_in_guest(void)
> +#define KVM_X86_DISABLE_EXITS_MWAIT (1 << 0)
> +#define KVM_X86_DISABLE_VALID_EXITS (KVM_X86_DISABLE_EXITS_MWAIT)
> +
> +static inline bool kvm_mwait_in_guest(struct kvm *kvm)
> {
> - return boot_cpu_has(X86_FEATURE_MWAIT) &&
> - !boot_cpu_has_bug(X86_BUG_MONITOR);
> + return kvm->arch.mwait_in_guest;
> }
>
> #endif
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 088c2c9..1065006 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -929,7 +929,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_S390_GS 140
> #define KVM_CAP_S390_AIS 141
> #define KVM_CAP_SPAPR_TCE_VFIO 142
> -#define KVM_CAP_X86_GUEST_MWAIT 143
> +#define KVM_CAP_X86_DISABLE_EXITS 143
> #define KVM_CAP_ARM_USER_IRQ 144
> #define KVM_CAP_S390_CMMA_MIGRATION 145
> #define KVM_CAP_PPC_FWNMI 146
> diff --git a/tools/include/uapi/linux/kvm.h b/tools/include/uapi/linux/kvm.h
> index 0fb5ef9..b13c257 100644
> --- a/tools/include/uapi/linux/kvm.h
> +++ b/tools/include/uapi/linux/kvm.h
> @@ -924,7 +924,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_S390_GS 140
> #define KVM_CAP_S390_AIS 141
> #define KVM_CAP_SPAPR_TCE_VFIO 142
> -#define KVM_CAP_X86_GUEST_MWAIT 143
> +#define KVM_CAP_X86_DISABLE_EXITS 143
> #define KVM_CAP_ARM_USER_IRQ 144
> #define KVM_CAP_S390_CMMA_MIGRATION 145
> #define KVM_CAP_PPC_FWNMI 146
> --
> 2.7.4
>