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

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Date:   Tue, 14 Mar 2017 14:58:24 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        "Gabriel L. Somlo" <gsomlo@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3] kvm: better MWAIT emulation for guests

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@...il.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.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@...il.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,