Re: [PATCH 1/3] KVM: Don't enable MWAIT in guest by default

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Nov 25, 2017 at 02:09:31PM +0100, Jan H. Schönherr wrote:
> 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.)
> 
> This restores the behavior from before Linux 4.12 commit 668fffa3f838
> ("kvm: better MWAIT emulation for guests") but keeps the option to
> enable MWAIT in guest for individual VMs.

As others pointed out, an interrupt will wake up the host CPU anyway.

Given that, what's the actual motivation here?

> Suggested-by: KarimAllah Ahmed <karahmed@amazon.de>
> Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
> ---
> Note: AMD code paths are only compile tested
> ---
>  Documentation/virtual/kvm/api.txt | 20 ++++++++++--------
>  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                | 44 ++++++++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/x86.h                | 35 ++-----------------------------
>  6 files changed, 64 insertions(+), 48 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index f670e4b..0ee812c 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4163,6 +4163,17 @@ 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_GUEST_MWAIT
> +
> +Architectures: x86
> +Parameters: none
> +Returns: 0 on success
> +
> +This capability indicates that a guest using memory monitoring instructions
> +(MWAIT/MWAITX) to stop a virtual CPU will not cause a VM exit. As such, time
> +spent while a 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. Other capabilities.
>  ----------------------
>  
> @@ -4275,15 +4286,6 @@ 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 b97726e..f7bcfaa 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -781,6 +781,8 @@ 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 1f3e7f2..ef1b320 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1253,7 +1253,7 @@ static void init_vmcb(struct vcpu_svm *svm)
>  	set_intercept(svm, INTERCEPT_WBINVD);
>  	set_intercept(svm, INTERCEPT_XSETBV);
>  
> -	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 1eb7053..a067735 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3635,13 +3635,11 @@ 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;
> @@ -5297,6 +5295,9 @@ 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 985a305..fe6627a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -67,6 +67,7 @@
>  #include <asm/pvclock.h>
>  #include <asm/div64.h>
>  #include <asm/irq_remapping.h>
> +#include <asm/mwait.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include "trace.h"
> @@ -2672,6 +2673,40 @@ static int msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs __user *user_msrs,
>  	return r;
>  }
>  
> +static bool kvm_mwait_in_guest_possible(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;
> +}
> +
>  int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  {
>  	int r;
> @@ -2726,7 +2761,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		r = KVM_CLOCK_TSC_STABLE;
>  		break;
>  	case KVM_CAP_X86_GUEST_MWAIT:
> -		r = kvm_mwait_in_guest();
> +		r = kvm_mwait_in_guest_possible();
>  		break;
>  	case KVM_CAP_X86_SMM:
>  		/* SMBASE is usually relocated above 1M on modern chipsets,
> @@ -4026,6 +4061,13 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  
>  		r = 0;
>  		break;
> +	case KVM_CAP_X86_GUEST_MWAIT:
> +		r = -EINVAL;
> +		if (kvm_mwait_in_guest_possible()) {
> +			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 d0b95b7..ed8e150 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"
> @@ -263,38 +261,9 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
>  	    __rem;						\
>  	 })
>  
> -static inline bool kvm_mwait_in_guest(void)
> +static inline bool kvm_mwait_in_guest(struct kvm *kvm)
>  {
> -	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;
> +	return kvm->arch.mwait_in_guest;
>  }
>  
>  #endif
> -- 
> 2.3.1.dirty