KVM: x86: Drop kvm_x86_ops.set_dr6() in favor of a new KVM_RUN flag

Instruct vendor code to load the guest's DR6 into hardware via a new
KVM_RUN flag, and remove kvm_x86_ops.set_dr6(), whose sole purpose was to
load vcpu->arch.dr6 into hardware when DR6 can be read/written directly
by the guest.

Note, TDX already WARNs on any run_flag being set, i.e. will yell if KVM
thinks DR6 needs to be reloaded.  TDX vCPUs force KVM_DEBUGREG_AUTO_SWITCH
and never clear the flag, i.e. should never observe KVM_RUN_LOAD_GUEST_DR6.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610232010.162191-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 8d50e3e0a19b9c1919004bf73b670b32829e72eb..9e0c37ea267e055a417b973eefbebe874b084342 100644 (file)
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -49,7 +49,6 @@ KVM_X86_OP(set_idt)
 KVM_X86_OP(get_gdt)
 KVM_X86_OP(set_gdt)
 KVM_X86_OP(sync_dirty_debug_regs)
-KVM_X86_OP(set_dr6)
 KVM_X86_OP(set_dr7)
 KVM_X86_OP(cache_reg)
 KVM_X86_OP(get_rflags)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 8d81684fa15d6848a42418469d3fc5a06abd3a37..4f7ee2dbf10e376a4b7170fcbeca5cd6570d7267 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1676,6 +1676,7 @@ static inline u16 kvm_lapic_irq_dest_mode(bool dest_mode_logical)
 
 enum kvm_x86_run_flags {
        KVM_RUN_FORCE_IMMEDIATE_EXIT    = BIT(0),
+       KVM_RUN_LOAD_GUEST_DR6          = BIT(1),
 };
 
 struct kvm_x86_ops {
@@ -1728,7 +1729,6 @@ struct kvm_x86_ops {
        void (*get_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
        void (*set_gdt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
        void (*sync_dirty_debug_regs)(struct kvm_vcpu *vcpu);
-       void (*set_dr6)(struct kvm_vcpu *vcpu, unsigned long value);
        void (*set_dr7)(struct kvm_vcpu *vcpu, unsigned long value);
        void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
        unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 83d1b62130b15d57a8c225b1f1ff1151fdd24e0a..be8c43049f4d398c3b0dc263fd1a862788c3c43e 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4438,10 +4438,13 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
        svm_hv_update_vp_id(svm->vmcb, vcpu);
 
        /*
-        * Run with all-zero DR6 unless needed, so that we can get the exact cause
-        * of a #DB.
+        * Run with all-zero DR6 unless the guest can write DR6 freely, so that
+        * KVM can get the exact cause of a #DB.  Note, loading guest DR6 from
+        * KVM's snapshot is only necessary when DR accesses won't exit.
         */
-       if (likely(!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)))
+       if (unlikely(run_flags & KVM_RUN_LOAD_GUEST_DR6))
+               svm_set_dr6(vcpu, vcpu->arch.dr6);
+       else if (likely(!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)))
                svm_set_dr6(vcpu, DR6_ACTIVE_LOW);
 
        clgi();
@@ -5252,7 +5255,6 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
        .set_idt = svm_set_idt,
        .get_gdt = svm_get_gdt,
        .set_gdt = svm_set_gdt,
-       .set_dr6 = svm_set_dr6,
        .set_dr7 = svm_set_dr7,
        .sync_dirty_debug_regs = svm_sync_dirty_debug_regs,
        .cache_reg = svm_cache_reg,

diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
index fef3e380370783478d80165ce3989eb9a71b29ee..c85cbce6d2f6efde46912ec9e9f3b1e4901a1ee1 100644 (file)
--- a/arch/x86/kvm/vmx/main.c
+++ b/arch/x86/kvm/vmx/main.c
@@ -489,14 +489,6 @@ static void vt_set_gdt(struct kvm_vcpu *vcpu, struct desc_ptr *dt)
        vmx_set_gdt(vcpu, dt);
 }
 
-static void vt_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
-{
-       if (is_td_vcpu(vcpu))
-               return;
-
-       vmx_set_dr6(vcpu, val);
-}
-
 static void vt_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
        if (is_td_vcpu(vcpu))
@@ -943,7 +935,6 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
        .set_idt = vt_op(set_idt),
        .get_gdt = vt_op(get_gdt),
        .set_gdt = vt_op(set_gdt),
-       .set_dr6 = vt_op(set_dr6),
        .set_dr7 = vt_op(set_dr7),
        .sync_dirty_debug_regs = vt_op(sync_dirty_debug_regs),
        .cache_reg = vt_op(cache_reg),

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a61a28944de64b52a97aea18e44a3613eee8e1fd..4ee6cc7968558e54c7ffc1cc0825fe4062f57d47 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5606,12 +5606,6 @@ void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
        set_debugreg(DR6_RESERVED, 6);
 }
 
-void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
-{
-       lockdep_assert_irqs_disabled();
-       set_debugreg(vcpu->arch.dr6, 6);
-}
-
 void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
        vmcs_writel(GUEST_DR7, val);
@@ -7370,6 +7364,9 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
                vmcs_writel(GUEST_RIP, vcpu->arch.regs[VCPU_REGS_RIP]);
        vcpu->arch.regs_dirty = 0;
 
+       if (run_flags & KVM_RUN_LOAD_GUEST_DR6)
+               set_debugreg(vcpu->arch.dr6, 6);
+
        /*
         * Refresh vmcs.HOST_CR3 if necessary.  This must be done immediately
         * prior to VM-Enter, as the kernel may load a new ASID (PCID) any time

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 07ff02eed399a8e8b94ecb79bdd311be78d53803..7fe3549bbd932e54abd12164a9281c38899c8a36 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11046,7 +11046,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                set_debugreg(vcpu->arch.eff_db[3], 3);
                /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
                if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
-                       kvm_x86_call(set_dr6)(vcpu, vcpu->arch.dr6);
+                       run_flags |= KVM_RUN_LOAD_GUEST_DR6;
        } else if (unlikely(hw_breakpoint_active())) {
                set_debugreg(0, 7);
        }