Re: [RFC PATCH v2 36/69] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 03/07/21 00:04, isaku.yamahata@xxxxxxxxx wrote:
> From: Sean Christopherson <sean.j.christopherson@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 | 11 ++++++++---
>   arch/x86/kvm/x86.c              |  3 ++-
>   2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 96e6cd95d884..7822b531a5e2 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -488,9 +488,14 @@ struct kvm_pmu {
>   struct kvm_pmu_ops;
>   
>   enum {
> -	KVM_DEBUGREG_BP_ENABLED = 1,
> -	KVM_DEBUGREG_WONT_EXIT = 2,
> -	KVM_DEBUGREG_RELOAD = 4,
> +	KVM_DEBUGREG_BP_ENABLED		= BIT(0),
> +	KVM_DEBUGREG_WONT_EXIT		= BIT(1),
> +	KVM_DEBUGREG_RELOAD		= BIT(2),
> +	/*
> +	 * Guest debug registers are saved/restored by hardware on exit from
> +	 * or enter guest. KVM needn't switch them.
> +	 */
> +	KVM_DEBUGREG_AUTO_SWITCH_GUEST	= BIT(3),

Maybe remove "_GUEST"?  Apart from that,

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Paolo

>   };
>   
>   struct kvm_mtrr_range {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4b436cae1732..f1d5e0a53640 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9441,7 +9441,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   	if (test_thread_flag(TIF_NEED_FPU_LOAD))
>   		switch_fpu_return();
>   
> -	if (unlikely(vcpu->arch.switch_db_regs)) {
> +	if (unlikely(vcpu->arch.switch_db_regs & ~KVM_DEBUGREG_AUTO_SWITCH_GUEST)) {
>   		set_debugreg(0, 7);
>   		set_debugreg(vcpu->arch.eff_db[0], 0);
>   		set_debugreg(vcpu->arch.eff_db[1], 1);
> @@ -9473,6 +9473,7 @@ 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_GUEST);
>   		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
>   		kvm_update_dr0123(vcpu);
>   		kvm_update_dr7(vcpu);