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

[ Upstream commit 80c64c7afea1da6a93ebe88d3d29d8a60377ef80 ]

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>
[sean: account for lack of vmx/main.c]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 0e5ae3b0c8676e59347b22fa3e34e8cc0145234c..c068565fe95474c38ef4010a54ef1cdb0fa94165 100644 (file)
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -47,7 +47,6 @@ 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 1383f5e5238a15fdccecbe99f81333e3b2033b72..c8fc4f2acf693eb54d79e7c5ffcf61aa78afb994 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1458,6 +1458,7 @@ 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 {
@@ -1504,7 +1505,6 @@ 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 dc8a1b72d8ec8b49cf4a17a4cf68d7624927c578..5a6bd9d5cceb77119b62a3dbd125eae3e7f27c27 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4052,10 +4052,13 @@ 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();
@@ -4822,7 +4825,6 @@ 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/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 382f4220068811810743af13c28a6d506a1a74c7..60d1ff3fca452c1a90ec492273b50fd1b19f5c14 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5530,12 +5530,6 @@ static void vmx_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
        set_debugreg(DR6_RESERVED, 6);
 }
 
-static void vmx_set_dr6(struct kvm_vcpu *vcpu, unsigned long val)
-{
-       lockdep_assert_irqs_disabled();
-       set_debugreg(vcpu->arch.dr6, 6);
-}
-
 static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 {
        vmcs_writel(GUEST_DR7, val);
@@ -7251,6 +7245,9 @@ static 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
@@ -8208,7 +8205,6 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
        .set_idt = vmx_set_idt,
        .get_gdt = vmx_get_gdt,
        .set_gdt = vmx_set_gdt,
-       .set_dr6 = vmx_set_dr6,
        .set_dr7 = vmx_set_dr7,
        .sync_dirty_debug_regs = vmx_sync_dirty_debug_regs,
        .cache_reg = vmx_cache_reg,

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 83e5e823cbae8e990f1f0c36c47bf10930a27f99..9d66830d594c49a6291577c537ba0a1b077156c6 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10833,7 +10833,7 @@ 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))
-                       static_call(kvm_x86_set_dr6)(vcpu, vcpu->arch.dr6);
+                       run_flags |= KVM_RUN_LOAD_GUEST_DR6;
        } else if (unlikely(hw_breakpoint_active())) {
                set_debugreg(0, 7);
        }