Re: [PATCH v19 088/130] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

[Binbin Wu]

On 2/26/2024 4:26 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.

Here "irrespective of any other flags" sounds like other flags will be 
ignored if KVM_DEBUGREG_AUTO_SWITCHED_GUEST is set.
But the code below doesn't align with it.

>    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 | 10 ++++++++--
>   arch/x86/kvm/vmx/tdx.c          |  1 +
>   arch/x86/kvm/x86.c              | 11 ++++++++---
>   3 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3ab85c3d86ee..a9df898c6fbd 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -610,8 +610,14 @@ 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 (DR0-3 and DR6) are saved/restored by hardware
> +	 * on exit from or enter to guest. KVM needn't switch them. Because DR7
> +	 * is cleared on exit from guest, DR7 need to be saved/restored.
> +	 */
> +	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 7aa9188f384d..ab7403a19c5d 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -586,6 +586,7 @@ 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 1b189e86a1f1..fb7597c22f31 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11013,7 +11013,7 @@ 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);
> @@ -11059,6 +11059,7 @@ 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);
> @@ -11071,8 +11072,12 @@ 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);

According to TDX module 1.5 ABI spec:
DR0-3, DR6 and DR7 are set to their architectural INIT value, why is 
only DR7 restored?


I found a discussion about it in
https://lore.kernel.org/kvm/Yk4iChT2sbtzSb8h@xxxxxxxxxx/

Sean mentioned:
"
The TDX module context switches the guest _and_ host debug registers.  
It restores
the host DRs because it needs to write _something_ to hide guest state, 
so it might
as well restore the host values.  The above was an optmization to avoid 
rewriting
all debug registers.
"

I have question on "so it might as well restore the host values".
Was it a guess/assumption or it was the fact?



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