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Subject: Re: [PATCH v2 03/21] KVM: x86: Don't check for code breakpoints
 when emulating on exception
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Oliver Upton <oupton@google.com>,
        Peter Shier <pshier@google.com>
Date:   Wed, 06 Jul 2022 14:43:55 +0300
In-Reply-To: <20220614204730.3359543-4-seanjc@google.com>
References: <20220614204730.3359543-1-seanjc@google.com>
         <20220614204730.3359543-4-seanjc@google.com>
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On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> Don't check for code breakpoints during instruction emulation if the
> emulation was triggered by exception interception.  Code breakpoints are
> the highest priority fault-like exception, and KVM only emulates on
> exceptions that are fault-like.  Thus, if hardware signaled a different
> exception, then the vCPU is already passed the stage of checking for
> hardware breakpoints.
> 
> This is likely a glorified nop in terms of functionality, and is more for
> clarification and is technically an optimization.  Intel's SDM explicitly
> states vmcs.GUEST_RFLAGS.RF on exception interception is the same as the
> value that would have been saved on the stack had the exception not been
> intercepted, i.e. will be '1' due to all fault-like exceptions setting RF
> to '1'.  AMD says "guest state saved ... is the processor state as of the
> moment the intercept triggers", but that begs the question, "when does
> the intercept trigger?".
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/x86.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2318a99139fa..c5db31b4bd6f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8364,8 +8364,24 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  }
>  EXPORT_SYMBOL_GPL(kvm_skip_emulated_instruction);
>  
> -static bool kvm_vcpu_check_code_breakpoint(struct kvm_vcpu *vcpu, int *r)
> +static bool kvm_vcpu_check_code_breakpoint(struct kvm_vcpu *vcpu,
> +					   int emulation_type, int *r)
>  {
> +	WARN_ON_ONCE(emulation_type & EMULTYPE_NO_DECODE);
> +
> +	/*
> +	 * Do not check for code breakpoints if hardware has already done the
> +	 * checks, as inferred from the emulation type.  On NO_DECODE and SKIP,
> +	 * the instruction has passed all exception checks, and all intercepted
> +	 * exceptions that trigger emulation have lower priority than code
> +	 * breakpoints, i.e. the fact that the intercepted exception occurred
> +	 * means any code breakpoints have already been serviced.
> +	 */
> +	if (emulation_type & (EMULTYPE_NO_DECODE | EMULTYPE_SKIP |
> +			      EMULTYPE_TRAP_UD | EMULTYPE_TRAP_UD_FORCED |
> +			      EMULTYPE_VMWARE_GP | EMULTYPE_PF))
> +		return false;
> +
>  	if (unlikely(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) &&
>  	    (vcpu->arch.guest_debug_dr7 & DR7_BP_EN_MASK)) {
>  		struct kvm_run *kvm_run = vcpu->run;
> @@ -8487,8 +8503,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  		 * are fault-like and are higher priority than any faults on
>  		 * the code fetch itself.
>  		 */
> -		if (!(emulation_type & EMULTYPE_SKIP) &&
> -		    kvm_vcpu_check_code_breakpoint(vcpu, &r))
> +		if (kvm_vcpu_check_code_breakpoint(vcpu, emulation_type, &r))
>  			return r;
>  
>  		r = x86_decode_emulated_instruction(vcpu, emulation_type,


Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky