Re: [PATCH v3 6/8] kvm: vmx: Extend VMX's #AC interceptor to handle split lock #AC happens in guest

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Feb 06, 2020 at 03:04:10PM +0800, Xiaoyao Li wrote:
> There are two types of #AC can be generated in Intel CPUs:
>  1. legacy alignment check #AC;
>  2. split lock #AC;
> 
> Legacy alignment check #AC can be injected to guest if guest has enabled
> alignemnet check.
> 
> when host enables split lock detectin, i.e., split_lock_detect != off,
> there will be an unexpected #AC in guest and intercepted by KVM because
> KVM doesn't virtualize this feature to guest and hardware value of
> MSR_TEST_CTRL.SLD bit stays unchanged when vcpu is running.
> 
> To handle this unexpected #AC, treat guest just like host usermode that
> calling handle_user_split_lock():
>  - If host is sld_warn, it warns and set TIF_SLD so that __switch_to_xtra()
>    does the MSR_TEST_CTRL.SLD bit switching when control transfer to/from
>    this vcpu.
>  - If host is sld_fatal, forward #AC to userspace, the similar as sending
>    SIGBUS.
> 
> Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---
> v3:
>  - Use handle_user_split_lock() to handle unexpected #AC in guest.
> ---
>  arch/x86/kvm/vmx/vmx.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c475fa2aaae0..822211975e6c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4557,6 +4557,12 @@ static int handle_machine_check(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> +static inline bool guest_cpu_alignment_check_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
> +	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
> +}
> +
>  static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4622,9 +4628,6 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  		return handle_rmode_exception(vcpu, ex_no, error_code);
>  
>  	switch (ex_no) {
> -	case AC_VECTOR:
> -		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> -		return 1;
>  	case DB_VECTOR:
>  		dr6 = vmcs_readl(EXIT_QUALIFICATION);
>  		if (!(vcpu->guest_debug &
> @@ -4653,6 +4656,28 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>  		kvm_run->debug.arch.exception = ex_no;
>  		break;
> +	case AC_VECTOR:
> +		/*
> +		 * Inject #AC back to guest only when guest enables legacy
> +		 * alignment check.


The comment should call out that checking split_lock_detect_enabled() is an
optimization.

		/*
		 * Reflect #AC to the guest if it's expecting the #AC, i.e. has
		 * legacy alignment check enabled.  Pre-check host split lock
		 * support to avoid the VMREADs needed to check legacy #AC,
		 * i.e. reflect the #AC if the only possible source is legacy
		 * alignment checks.
		 */

> +		 * Otherwise, it must be an unexpected split lock #AC of guest
> +		 * since hardware SPLIT_LOCK_DETECT bit keeps unchanged set
> +		 * when vcpu is running. In this case, treat guest the same as
> +		 * user space application that calls handle_user_split_lock():
> +		 *  - If sld_state = sld_warn, it sets TIF_SLD and disables SLD
> +		 *    for this vcpu thread.
> +		 *  - If sld_state = sld_fatal, we forward #AC to userspace,
> +		 *    similar as sending SIGBUS.

I'd prefer to avoid talking about sld_state at all and instead keep those
details in handle_user_split_lock().


> +		 */
> +		if (!split_lock_detect_enabled() ||
> +		    guest_cpu_alignment_check_enabled(vcpu)) {
> +			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> +			return 1;
> +		}

Something like:

		/*
		 * Forward the #AC to userspace if kernel policy does not allow
		 * temporarily disabling split lock detection.
		 */

> +		if (handle_user_split_lock(kvm_rip_read(vcpu)))
> +			return 1;
> +		/* fall through */
>  	default:
>  		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
>  		kvm_run->ex.exception = ex_no;
> -- 
> 2.23.0
>