Messages in this thread | | | Date | Sun, 7 Apr 2024 18:52:44 +0800 | Subject | Re: [PATCH v19 088/130] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior | From | Binbin Wu <> |
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On 2/26/2024 4:26 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.
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@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 | 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@google.com/
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());
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