Re: [PATCH v3] kvm: better MWAIT emulation for guests

Radim Krčmář
2017-03-14T13:58:24Z

2017-03-14 01:44+0200, Michael S. Tsirkin:
> 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.
>
> Reported-by: "Gabriel L. Somlo" <gsomlo@gmail.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> Note: SVM bits are untested at this point. Seems pretty
> obvious though.
>
> changes from v2:
> - add a capability to allow host userspace to detect new kernels
> - more documentation to clarify the semantics of the feature flag
> and why it's useful
> - svm support as suggested by Radim
>
> changes from v1:
> - typo fix resulting in rest of leaf flags being overwritten
> Reported by: Wanpeng Li <kernellwp@gmail.com>
> - updated commit log with data about guests helped by this feature
> - better document differences between mwait and halt for guests
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> @@ -4135,11 +4135,11 @@ available, means that that the kernel can support guests using the
> radix MMU defined in Power ISA V3.00 (as implemented in the POWER9
> processor).
>
> -8.4 KVM_CAP_PPC_HASH_MMU_V3

This patch should not not remove the PPC capability from docs.

(The right name is KVM_CAP_PPC_HASH_V3, but that is for another patch.)

> +8.5 KVM_CAP_X86_GUEST_MWAIT
>
> -Architectures: ppc
> +Architectures: x86
>
> -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.
> +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/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 = !!this_cpu_has(X86_FEATURE_MWAIT);

this_cpu_has already returns bool, so !! is not needed.

I can fix both while applying.

> + 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,