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

Binbin Wu
2022-11-16T02:41:45Z

On 10/30/2022 2:23 PM, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> 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@intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Co-developed-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> 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());