linux-kernel - Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

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Date:	Wed, 18 Jun 2014 15:09:00 -0400
From:	Bandan Das <bsd@...hat.com>
To:	"Gabriel L. Somlo" <gsomlo@...il.com>
Cc:	Eric Northup <digitaleric@...gle.com>,
	Nadav Amit <namit@...technion.ac.il>,
	Paolo Bonzini <pbonzini@...hat.com>, gleb@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>, joro@...tes.org, mst@...hat.com,
	agraf@...e.de
Subject: Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

"Gabriel L. Somlo" <gsomlo@...il.com> writes:

> On Wed, Jun 18, 2014 at 10:59:14AM -0700, Eric Northup wrote:
>> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@...technion.ac.il> wrote:
>> > mwait and monitor are currently handled as nop. Considering this behavior, they
>> > should still be handled correctly, i.e., check execution conditions and generate
>> > exceptions when required. mwait and monitor may also be executed in real-mode
>> > and are not handled in that case.  This patch performs the emulation of
>> > monitor-mwait according to Intel SDM (other than checking whether interrupt can
>> > be used as a break event).
>> >
>> > Signed-off-by: Nadav Amit <namit@...technion.ac.il>
>
> How about this instead (details in the commit log below) ? Please let
> me know what you think, and if you'd prefer me to send it out as a
> separate patch rather than a reply to this thread.

I am not saying we should "punish" guests that don't do the right thing.
But if we can't emulate it the right way, we emulate it  ? 
And the fact that a workaround exists for the guest is all the more better :)

> Thanks,
> --Gabriel
>
>
> From 0375a0aceb54cdbc26a6c0e5b43c46324f830ec3 Mon Sep 17 00:00:00 2001
> From: "Gabriel L. Somlo" <somlo@....edu>
> Date: Wed, 18 Jun 2014 14:39:15 -0400
> Subject: [PATCH] kvm: x86: revert "emulate monitor and mwait instructions as nop"
>
> This reverts commit 87c00572ba05aa8c9db118da75c608f47eb10b9e.
>
> OS X <= 10.7.* are the only known guests which realistically required
> this functionality. As it turns out, OS X can be told to forego using
> monitor/mwait by passing it "idlehalt=0" as a kernel argument, so we're
> better off removing this hack from KVM altogether.
>
> Signed-off-by: Gabriel L. Somlo <somlo@....edu>
> ---
>  arch/x86/kvm/cpuid.c |  2 --
>  arch/x86/kvm/svm.c   |  8 +++-----
>  arch/x86/kvm/vmx.c   | 10 ++++------
>  3 files changed, 7 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 38a0afe..17b42fa 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -283,8 +283,6 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  		0 /* Reserved */ | f_lm | F(3DNOWEXT) | F(3DNOW);
>  	/* cpuid 1.ecx */
>  	const u32 kvm_supported_word4_x86_features =
> -		/* NOTE: MONITOR (and MWAIT) are emulated as NOP,
> -		 * but *not* advertised to guests via CPUID ! */
>  		F(XMM3) | F(PCLMULQDQ) | 0 /* DTES64, MONITOR */ |
>  		0 /* DS-CPL, VMX, SMX, EST */ |
>  		0 /* TM2 */ | F(SSSE3) | 0 /* CNXT-ID */ | 0 /* Reserved */ |
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ec8366c..0e8ef20 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3274,7 +3274,7 @@ static int pause_interception(struct vcpu_svm *svm)
>  	return 1;
>  }
>  
> -static int nop_interception(struct vcpu_svm *svm)
> +static int invalid_op_interception(struct vcpu_svm *svm)
>  {
>  	skip_emulated_instruction(&(svm->vcpu));
>  	return 1;
> @@ -3282,14 +3282,12 @@ static int nop_interception(struct vcpu_svm *svm)
>  
>  static int monitor_interception(struct vcpu_svm *svm)
>  {
> -	printk_once(KERN_WARNING "kvm: MONITOR instruction emulated as NOP!\n");
> -	return nop_interception(svm);
> +	return invalid_op_interception(svm);
>  }
>  
>  static int mwait_interception(struct vcpu_svm *svm)
>  {
> -	printk_once(KERN_WARNING "kvm: MWAIT instruction emulated as NOP!\n");
> -	return nop_interception(svm);
> +	return invalid_op_interception(svm);
>  }
>  
>  static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 801332e..577c7df 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5672,22 +5672,20 @@ static int handle_pause(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> -static int handle_nop(struct kvm_vcpu *vcpu)
> +static int handle_invalid_op(struct kvm_vcpu *vcpu)
>  {
> -	skip_emulated_instruction(vcpu);
> +	kvm_queue_exception(vcpu, UD_VECTOR);
>  	return 1;
>  }
>  
>  static int handle_mwait(struct kvm_vcpu *vcpu)
>  {
> -	printk_once(KERN_WARNING "kvm: MWAIT instruction emulated as NOP!\n");
> -	return handle_nop(vcpu);
> +	return handle_invalid_op(vcpu);
>  }
>  
>  static int handle_monitor(struct kvm_vcpu *vcpu)
>  {
> -	printk_once(KERN_WARNING "kvm: MONITOR instruction emulated as NOP!\n");
> -	return handle_nop(vcpu);
> +	return handle_invalid_op(vcpu);
>  }
>  
>  /*
--
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