Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

Gabriel L. Somlo
2017-03-15T23:35:34Z

On Wed, Mar 15, 2017 at 11:22:18PM +0200, Michael S. Tsirkin wrote:
> Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem:
> unless explicitly provided with kernel command line argument
> "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability,
> without checking CPUID.
>
> We currently emulate that as a NOP but on VMX we can do better: let
> guest stop the CPU until timer, IPI or memory change. CPU will be busy
> but that isn't any worse than a NOP emulation.
>
> Note that mwait within guests is not the same as on real hardware
> because halt causes an exit while mwait doesn't. For this reason it
> might not be a good idea to use the regular MWAIT flag in CPUID to
> signal this capability. Add a flag in the hypervisor leaf instead.
>
> Additionally, we add a capability for QEMU - e.g. if it knows there's an
> isolated CPU dedicated for the VCPU it can set the standard MWAIT flag
> to improve guest behaviour.

Same behavior (on the mac pro 1,1 running F22 with custom-compiled
kernel from kvm git master, plus this patch on top).

The OS X 10.7 kernel hangs (or at least progresses extremely slowly)
on boot, does not bring up guest graphical interface within the first
10 minutes that I waited for it. That, in contrast with the default
nop-based emulation where the guest comes up within 30 seconds.

I will run another round of tests on a newer Mac (4-year-old macbook
air) and report back tomorrow.

Going off on a tangent, why would encouraging otherwise well-behaved
guests (like linux ones, for example) to use MWAIT be desirable to
begin with ? Is it a matter of minimizing the overhead associated with
exiting and re-entering L1 ? Because if so, AFAIR staying inside L1 and
running guest-mode MWAIT in a tight loop will actually waste the host
CPU without the opportunity to yield to some other L0 thread. Sorry if
I fell into the middle of an ongoing conversation on this and missed
most of the relevant context, in which case please feel free to ignore
me... :)

Thanks,
--G

>
> Reported-by: "Gabriel L. Somlo" <gsomlo@gmail.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> This is for Gabriel's testing only. A bit rushed so untested.
>
> 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 | 2 --
> arch/x86/kvm/vmx.c | 6 ++++--
> arch/x86/kvm/x86.c | 3 +++
> arch/x86/kvm/x86.h | 28 ++++++++++++++++++++++++++++
> include/uapi/linux/kvm.h | 1 +
> 9 files changed, 55 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 3c248f7..6ee2e43 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4147,3 +4147,12 @@ This capability, if KVM_CHECK_EXTENSION indicates that it is
> available, means that that the kernel can support guests using the
> hashed page table MMU defined in Power ISA V3.00 (as implemented in
> the POWER9 processor), including in-memory segment tables.
> +
> +8.5 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 d1efe2c..18e53bc 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1198,8 +1198,6 @@ 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);
>
> control->iopm_base_pa = iopm_base;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 98e82ee..ea0c96a 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 1faf620..8c74fff 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2684,6 +2684,9 @@ 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..a2d8964 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 +214,30 @@ 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;
> +
> + if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> + 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 f51d508..8b6bc06 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -883,6 +883,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_PPC_MMU_RADIX 134
> #define KVM_CAP_PPC_MMU_HASH_V3 135
> #define KVM_CAP_IMMEDIATE_EXIT 136
> +#define KVM_CAP_X86_GUEST_MWAIT 137
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> MST