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

Gabriel L. Somlo
2014-06-18T18:46:07Z

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@cs.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@cs.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.

Thanks,
--Gabriel

>From 0375a0aceb54cdbc26a6c0e5b43c46324f830ec3 Mon Sep 17 00:00:00 2001
From: "Gabriel L. Somlo" <somlo@cmu.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@cmu.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);
}

/*
--
1.9.3