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