Re: [PATCH v4 17/17] kvm: vmx: Emulate TEST_CTL MSR

[Xiaoyao Li <xiaoyao.li@linux.intel.com>]

On Mon, 2019-03-11 at 14:31 +0100, Paolo Bonzini wrote:
> On 09/03/19 03:31, Xiaoyao Li wrote:
> > Hi, Paolo,
> > 
> > Do you have any comments on this patch?
> > 
> > We are preparing v5 patches for split lock detection, if you have any
> > comments
> > about this one, please let me know.
> 
> No, my only comment is that it should be placed _before_ the other two
> for bisectability.  I think I have already sent that small remark.
> 
> Thanks,
> 
> Paolo

I cannot find the small remark you sent before. Maybe I missed something.
But I'am confused why it should be placed _before_ the other two. This patch
just use the vmx->core_capability that defined it the previous patch.

> > Thanks,
> > Xiaoyao
> > 
> > On Fri, 2019-03-01 at 18:45 -0800, Fenghua Yu wrote:
> > > From: Xiaoyao Li <xiaoyao.li@linux.intel.com>
> > > 
> > > 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 Software Developer's Manual
> > > for more detailed information on the MSR and the split lock bit.
> > > 
> > > 1. Since the kernel chooses to enable AC split lock by default, which
> > > means if we don't emulate TEST_CTL MSR for guest, guest will run with
> > > this feature enable while does not known it. Thus existing guests with
> > > buggy firmware (like OVMF) and old kernels having the cross cache line
> > > issues will fail the boot due to #AC.
> > > 
> > > So we should emulate TEST_CTL MSR, and set it zero to disable AC split
> > > lock by default. Whether and when to enable it is left to guest firmware
> > > and guest kernel.
> > > 
> > > 2. Host and guest can enable AC split lock independently, so using
> > > msr autoload to switch it during VM entry/exit.
> > > 
> > > Signed-off-by: Xiaoyao Li <xiaoyao.li@linux.intel.com>
> > > Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 35 +++++++++++++++++++++++++++++++++++
> > >  arch/x86/kvm/vmx/vmx.h |  1 +
> > >  2 files changed, 36 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 3e03c6e1e558..c0c5e8621afa 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -1659,6 +1659,12 @@ 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 &&
> > > +		    !(vmx->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);
> > > @@ -1805,6 +1811,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu,
> > > struct
> > > msr_data *msr_info)
> > >  	u32 index;
> > >  
> > >  	switch (msr_index) {
> > > +	case MSR_TEST_CTL:
> > > +		if (!(vmx->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;
> > > @@ -4108,6 +4122,9 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
> > >  
> > >  	vmx->arch_capabilities = kvm_get_arch_capabilities();
> > >  
> > > +	/* 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 */
> > > @@ -4145,6 +4162,7 @@ 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();
> > > @@ -6344,6 +6362,21 @@ 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 (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
> > > +		return;
> > > +
> > > +	rdmsrl(MSR_TEST_CTL, host_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);
> > > @@ -6585,6 +6618,8 @@ 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 cc22379991f3..e8831609c6c3 100644
> > > --- a/arch/x86/kvm/vmx/vmx.h
> > > +++ b/arch/x86/kvm/vmx/vmx.h
> > > @@ -191,6 +191,7 @@ struct vcpu_vmx {
> > >  	u64		      msr_guest_kernel_gs_base;
> > >  #endif
> > >  
> > > +	u64		      msr_test_ctl;
> > >  	u64		      core_capability;
> > >  	u64		      arch_capabilities;
> > >  	u64		      spec_ctrl;
> 
>