linux-kernel - Re: [RFC PATCH v5 074/104] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

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Message-ID: <ea136ac6-53cf-cdc5-a741-acfb437819b1@redhat.com>
Date:   Tue, 5 Apr 2022 17:32:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 074/104] KVM: x86: Add a switch_db_regs flag to
 handle TDX's auto-switched behavior

On 3/4/22 20:49, isaku.yamahata@...el.com wrote:
>   	vcpu->arch.efer = EFER_SCE | EFER_LME | EFER_LMA | EFER_NX;
>   
> +	vcpu->arch.switch_db_regs = KVM_DEBUGREG_AUTO_SWITCH;
>   	vcpu->arch.cr0_guest_owned_bits = -1ul;
>   	vcpu->arch.cr4_guest_owned_bits = -1ul;
>   
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 45e8a02e99bf..89d04cd64cd0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10084,7 +10084,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   	if (vcpu->arch.guest_fpu.xfd_err)
>   		wrmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>   
> -	if (unlikely(vcpu->arch.switch_db_regs)) {
> +	if (unlikely(vcpu->arch.switch_db_regs & ~KVM_DEBUGREG_AUTO_SWITCH)) {
>   		set_debugreg(0, 7);
>   		set_debugreg(vcpu->arch.eff_db[0], 0);
>   		set_debugreg(vcpu->arch.eff_db[1], 1);

I'm confused.  I'm probably missing something obvious, but where does
KVM_DEBUGREG_AUTO_SWITCH affect the behavior of KVM?  vcpu_enter_guest
would still write %db0-%db3 if KVM_DEBUGREG_BP_ENABLED is set, for
example.

Do you mean:

	if (unlikely(vcpu->arch.switch_db_regs) &&
	    !unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH)) {

?

Paolo
  
> @@ -10126,6 +10126,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   	 */
>   	if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
>   		WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);
> +		WARN_ON(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH);
>   		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
>   		kvm_update_dr0123(vcpu);
>   		kvm_update_dr7(vcpu);