Re: [PATCH v10 077/108] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

[Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>]

On 10/30/2022 2:23 PM, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
> Add a flag, KVM_DEBUGREG_AUTO_SWITCHED_GUEST, to skip saving/restoring DRs
> irrespective of any other flags.  TDX-SEAM unconditionally saves and
> restores guest DRs and reset to architectural INIT state on TD exit.
> So, KVM needs to save host DRs before TD enter without restoring guest DRs
> and restore host DRs after TD exit.
>
> Opportunistically convert the KVM_DEBUGREG_* definitions to use BIT().
>
> Reported-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Co-developed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>   arch/x86/include/asm/kvm_host.h |  9 +++++++--
>   arch/x86/kvm/vmx/tdx.c          |  1 +
>   arch/x86/kvm/x86.c              | 11 ++++++++---
>   3 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index fdb00d96e954..082e94f78c66 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -583,8 +583,13 @@ struct kvm_pmu {
>   struct kvm_pmu_ops;
>   
>   enum {
> -	KVM_DEBUGREG_BP_ENABLED = 1,
> -	KVM_DEBUGREG_WONT_EXIT = 2,
> +	KVM_DEBUGREG_BP_ENABLED		= BIT(0),
> +	KVM_DEBUGREG_WONT_EXIT		= BIT(1),
> +	/*
> +	 * Guest debug registers are saved/restored by hardware on exit from
> +	 * or enter guest. KVM needn't switch them.
> +	 */
> +	KVM_DEBUGREG_AUTO_SWITCH	= BIT(2),
>   };
>   
>   struct kvm_mtrr_range {
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index fc4de83a2df8..57767ef3353b 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -429,6 +429,7 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
>   
>   	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 4d4b71c4cdb1..ad7b227b68dd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10779,7 +10779,7 @@ 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);
> @@ -10822,6 +10822,7 @@ 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);
> @@ -10834,8 +10835,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   	 * care about the messed up debug address registers. But if
>   	 * we have some of them active, restore the old state.
>   	 */
> -	if (hw_breakpoint_active())
> -		hw_breakpoint_restore();
> +	if (hw_breakpoint_active()) {
> +		if (!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))
> +			hw_breakpoint_restore();
> +		else
> +			set_debugreg(__this_cpu_read(cpu_dr7), 7);

Why only restore dr7 when TD exit?

According to the commit message, dr0~dr3 are also reset to architectural 
INIT value on TD exit.



> +	}
>   
>   	vcpu->arch.last_vmentry_cpu = vcpu->cpu;
>   	vcpu->arch.last_guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());