[PATCH V2 12/12] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

Adrian Hunter
2025-01-29T10:00:35Z

From: Isaku Yamahata <isaku.yamahata@intel.com>

Add a flag KVM_DEBUGREG_AUTO_SWITCH to skip saving/restoring guest
DRs.

TDX-SEAM unconditionally saves/restores guest DRs on TD exit/enter,
and resets DRs to architectural INIT state on TD exit. Use the new
flag KVM_DEBUGREG_AUTO_SWITCH to indicate that KVM doesn't need to
save/restore guest DRs. KVM still needs to restore host DRs after TD
exit if there are active breakpoints in the host, which is covered by
the existing code.

MOV-DR exiting is always cleared for TDX guests, so the handler for DR
access is never called, and KVM_DEBUGREG_WONT_EXIT is never set. Add
a warning if both KVM_DEBUGREG_WONT_EXIT and KVM_DEBUGREG_AUTO_SWITCH
are set.

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>
[binbin: rework changelog]
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20241210004946.3718496-2-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
TD vcpu enter/exit v2:
- Moved from TDX "the rest" to "TD vcpu enter/exit"

TDX "the rest" v1:
- Update the comment about KVM_DEBUGREG_AUTO_SWITCH.
- Check explicitly KVM_DEBUGREG_AUTO_SWITCH is not set in switch_db_regs
before restoring guest DRs, because KVM_DEBUGREG_BP_ENABLED could be set
by userspace. (Paolo)
https://lore.kernel.org/lkml/ea136ac6-53cf-cdc5-a741-acfb437819b1@redhat.com/
- Fix the issue that host DRs are not restored in v19 (Binbin)
https://lore.kernel.org/kvm/20240413002026.GP3039520@ls.amr.corp.intel.com/
- Update the changelog a bit.
---
arch/x86/include/asm/kvm_host.h | 11 +++++++++--
arch/x86/kvm/vmx/tdx.c | 1 +
arch/x86/kvm/x86.c | 4 +++-
3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e557a441fade..bcfd89c28308 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -606,8 +606,15 @@ 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, DR6 and DR7) are saved/restored by
+ * hardware on exit from or enter to guest. KVM needn't switch them.
+ * DR0-3, DR6 and DR7 are set to their architectural INIT value on VM
+ * exit, host values need to be restored.
+ */
+ KVM_DEBUGREG_AUTO_SWITCH = BIT(2),
};

struct kvm_mtrr {
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 0bce00415f42..0863bdaf761a 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -652,6 +652,7 @@ 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 15447fe7687c..b023283e7ed4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10977,7 +10977,8 @@ 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 &&
+ !(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);
@@ -11024,6 +11025,7 @@ 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);
kvm_x86_call(sync_dirty_debug_regs)(vcpu);
kvm_update_dr0123(vcpu);
kvm_update_dr7(vcpu);
--
2.43.0