Subject: Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL

From: Xiaoyao Li <[email-masked]>
Date: Mon, 08 Apr 2019 16:54:15 +0800

On Thu, 2019-04-04 at 07:44 -0700, Sean Christopherson wrote:
> On Wed, Apr 03, 2019 at 02:21:58PM -0700, Fenghua Yu wrote:
> > From: Xiaoyao Li <[email-masked]>
> > 
> > A control bit (bit 29) in TEST_CTL MSR 0x33 will be introduced in
> > future x86 processors. When bit 29 is set, the processor causes #AC
> > exception for split locked accesses at all CPL.
> > 
> > Please check the latest Intel 64 and IA-32 Architectures Software
> > Developer's Manual for more detailed information on the MSR and
> > the split lock bit.
> > 
> > This patch emulate MSR TEST_CTL with vmx->msr_test_ctl and does the
> > following:
> > 1. As MSR TEST_CTL of guest is emulated, enable the related bits
> > in CORE_CAPABILITY to corretly report this feature to guest.
> 
> s/corretly/correctly

will correct it. thanks.

> > 
> > 2. Differentiate MSR TEST_CTL between host and guest.
> > 
> > Signed-off-by: Xiaoyao Li <[email-masked]>
> > Signed-off-by: Fenghua Yu <[email-masked]>
> > Acked-by: Paolo Bonzini <[email-masked]>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 35 +++++++++++++++++++++++++++++++++++
> >  arch/x86/kvm/vmx/vmx.h |  1 +
> >  arch/x86/kvm/x86.c     | 17 ++++++++++++++++-
> >  3 files changed, 52 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index ab432a930ae8..309ccf593f0d 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1663,6 +1663,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct
> > msr_data *msr_info)
> >  	u32 index;
> >  
> >  	switch (msr_info->index) {
> > +	case MSR_TEST_CTL:
> > +		if (!msr_info->host_initiated &&
> > +		    !(vcpu->arch.core_capability & CORE_CAP_SPLIT_LOCK_DETECT))
> > +			return 1;
> > +		msr_info->data = vmx->msr_test_ctl;
> > +		break;
> >  #ifdef CONFIG_X86_64
> >  	case MSR_FS_BASE:
> >  		msr_info->data = vmcs_readl(GUEST_FS_BASE);
> > @@ -1797,6 +1803,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct
> > msr_data *msr_info)
> >  	u32 index;
> >  
> >  	switch (msr_index) {
> > +	case MSR_TEST_CTL:
> > +		if (!(vcpu->arch.core_capability & CORE_CAP_SPLIT_LOCK_DETECT))
> > +			return 1;
> > +
> > +		if (data & ~TEST_CTL_ENABLE_SPLIT_LOCK_DETECT)
> > +			return 1;
> > +		vmx->msr_test_ctl = data;
> > +		break;
> >  	case MSR_EFER:
> >  		ret = kvm_set_msr_common(vcpu, msr_info);
> >  		break;
> > @@ -4077,6 +4091,9 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
> >  		++vmx->nmsrs;
> >  	}
> >  
> > +	/* disable AC split lock by default */
> > +	vmx->msr_test_ctl = 0;
> > +
> >  	vm_exit_controls_init(vmx, vmx_vmexit_ctrl());
> >  
> >  	/* 22.2.1, 20.8.1 */
> > @@ -4114,6 +4131,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool
> > init_event)
> >  
> >  	vmx->rmode.vm86_active = 0;
> >  	vmx->spec_ctrl = 0;
> > +	vmx->msr_test_ctl = 0;
> >  
> >  	vcpu->arch.microcode_version = 0x100000000ULL;
> >  	vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
> > @@ -6313,6 +6331,21 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx
> > *vmx)
> >  					msrs[i].host, false);
> >  }
> >  
> > +static void atomic_switch_msr_test_ctl(struct vcpu_vmx *vmx)
> > +{
> > +	u64 host_msr_test_ctl;
> > +
> > +	/* if TEST_CTL MSR doesn't exist on the hardware, do nothing */
> > +	if (rdmsrl_safe(MSR_TEST_CTL, &host_msr_test_ctl))
> > +		return;
> 
> This adds a RDMSR on every VM-Enter, and a fault on CPUs that don't
> support MSR_TEST_CTL.  Ideally the kernel would cache MSR_TEST_CTL and
> expose a helper that returns a boolean to indicate the existence of the
> MSRs along with the current value.  Racing with split_lock_detect_store()
> is ok since this code runs with interrupts disabled, i.e. will block
> split_lock_detect_store() until after VM-Exit.
> 
> Paolo, can you weigh in with your thoughts?  I'm surprised you acked
> this patch given your earlier comment:
> 
> https://patchwork.kernel.org/patch/10413779/#21892723

In v4 patchset, it checks boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) first in
atomic_switch_msr_test_ctl().

In v5 patchset, considering that there is another bit (bit 31) in MSR_TEST_CTL,
and !boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) cannot guarantee there is no
MSR_TEST_CTL in hardware. So I changed it to rdmsrl_safe() in v5.
It's my fault that I didn't point it out in the changelog in v5 patchset.

Considering the overhead of rdmsr every VMENTRY. I will use a percpu variable
msr_test_ctl_cache based on Thomas's comment to cache the value of host's
MSR_TEST_CTL.

> > +
> > +	if (host_msr_test_ctl == vmx->msr_test_ctl)
> > +		clear_atomic_switch_msr(vmx, MSR_TEST_CTL);
> > +	else
> > +		add_atomic_switch_msr(vmx, MSR_TEST_CTL, vmx->msr_test_ctl,
> > +				      host_msr_test_ctl, false);
> > +}
> > +
> >  static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val)
> >  {
> >  	vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val);
> > @@ -6419,6 +6452,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  
> >  	atomic_switch_perf_msrs(vmx);
> >  
> > +	atomic_switch_msr_test_ctl(vmx);
> > +
> >  	vmx_update_hv_timer(vcpu);
> >  
> >  	/*
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index a1e00d0a2482..6091a8b9de74 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -190,6 +190,7 @@ struct vcpu_vmx {
> >  	u64		      msr_guest_kernel_gs_base;
> >  #endif
> >  
> > +	u64		      msr_test_ctl;
> >  	u64		      spec_ctrl;
> >  
> >  	u32 vm_entry_controls_shadow;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4459115eb0ec..e93c2f620cdb 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1229,7 +1229,22 @@ EXPORT_SYMBOL_GPL(kvm_get_arch_capabilities);
> >  
> >  u64 kvm_get_core_capability(void)
> >  {
> > -	return 0;
> > +	u64 data;
> > +
> > +	rdmsrl_safe(MSR_IA32_CORE_CAPABILITY, &data);
> > +
> > +	/* mask non-virtualizable functions */
> > +	data &= CORE_CAP_SPLIT_LOCK_DETECT;
> > +
> > +	/*
> > +	 * There will be a list of FMS values that have split lock detection
> > +	 * but lack the CORE CAPABILITY MSR. In this case, set
> > +	 * CORE_CAP_SPLIT_LOCK_DETECT since we emulate MSR CORE_CAPABILITY.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
> > +		data |= CORE_CAP_SPLIT_LOCK_DETECT;
> > +
> > +	return data;
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_get_core_capability);
> >  
> > -- 
> > 2.19.1
> >