Re: [PATCH v5 2/8] KVM: arm64: Make vcpu_{read,write}_sys_reg available to HYP code

[Marc Zyngier]

On Tue, 14 Jul 2026 16:32:54 +0100,
Fuad Tabba <fuad.tabba@xxxxxxxxx> wrote:
> 
> On Tue, 14 Jul 2026 at 16:19, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> >
> > On Tue, 14 Jul 2026 11:15:55 +0100,
> > Fuad Tabba <fuad.tabba@xxxxxxxxx> wrote:
> > >
> > > The vcpu_{read,write}_sys_reg() accessors are only valid on a VHE host,
> > > so helpers built on them such as kvm_vcpu_set_be()/kvm_vcpu_is_be()
> > > cannot be shared with hyp code. exception.c already wraps them in local
> > > helpers that pick the host- or hyp-side accessor via has_vhe().
> > >
> > > Rename the host-only implementations to __vcpu_{read,write}_sysreg_vhe()
> >
> > I'm a bit puzzled by this. There is nothing that makes these functions
> > VHE-specific. Look at where they are called from: plenty of non-VHE
> > uses. These helpers are the canonical accessors for any system
> > register, and they don't cater for any particular mode.
> 
> I might have misunderstood what Oliver asked me to do here [1]:
> > Can you instead name the wrappers vcpu_{read,write}_sys_reg() and rename
> > the current implementations, like __vcpu_{read,write}_sysreg_vhe()?
> 
> Since they're only used and gated by has_vhe(), they're vhe-specific?

But they factually aren't VHE specific. Look at sys_reg.c, for
example. The whole point is that they abstract where the registers are
located, irrespective of the KVM mode.

The code in exception.c is actually a local specialisation of this
code for the nVHE code not to explode. And the exact same effect could
be achieved without repainting the whole thing and renaming *perfectly
named* accessors! ;-)

Something like below (compile-tested only).

	M.

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 9d9e7674d45fe..5ea708ff29436 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -526,6 +526,14 @@ static inline unsigned long kvm_vcpu_get_mpidr_aff(struct kvm_vcpu *vcpu)
 	return __vcpu_sys_reg(vcpu, MPIDR_EL1) & MPIDR_HWID_BITMASK;
 }
 
+#if   defined (__KVM_NVHE_HYPERVISOR__)
+#define vcpu_read_sys_reg(v, r)		__vcpu_sys_reg(v, r)
+#define vcpu_write_sys_reg(v, x, r)	__vcpu_assign_sys_reg(v, x, r)
+#elif defined (__KVM_VHE_HYPERVISOR__)
+#define vcpu_read_sys_reg(v, r)		vcpu_read_sys_reg(v, r)
+#define vcpu_write_sys_reg(v, x, r)	vcpu_write_sys_reg(v, x, r)
+#endif
+
 static inline void kvm_vcpu_set_be(struct kvm_vcpu *vcpu)
 {
 	if (vcpu_mode_is_32bit(vcpu)) {
diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
index bef40ddb16dbc..754e2dc1df54a 100644
--- a/arch/arm64/kvm/hyp/exception.c
+++ b/arch/arm64/kvm/hyp/exception.c
@@ -20,22 +20,6 @@
 #error Hypervisor code only!
 #endif
 
-static inline u64 __vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
-{
-	if (has_vhe())
-		return vcpu_read_sys_reg(vcpu, reg);
-
-	return __vcpu_sys_reg(vcpu, reg);
-}
-
-static inline void __vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
-{
-	if (has_vhe())
-		vcpu_write_sys_reg(vcpu, val, reg);
-	else
-		__vcpu_assign_sys_reg(vcpu, reg, val);
-}
-
 static void __vcpu_write_spsr(struct kvm_vcpu *vcpu, unsigned long target_mode,
 			      u64 val)
 {
@@ -101,14 +85,14 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
 
 	switch (target_mode) {
 	case PSR_MODE_EL1h:
-		vbar = __vcpu_read_sys_reg(vcpu, VBAR_EL1);
-		sctlr = __vcpu_read_sys_reg(vcpu, SCTLR_EL1);
-		__vcpu_write_sys_reg(vcpu, *vcpu_pc(vcpu), ELR_EL1);
+		vbar = vcpu_read_sys_reg(vcpu, VBAR_EL1);
+		sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
+		vcpu_write_sys_reg(vcpu, *vcpu_pc(vcpu), ELR_EL1);
 		break;
 	case PSR_MODE_EL2h:
-		vbar = __vcpu_read_sys_reg(vcpu, VBAR_EL2);
-		sctlr = __vcpu_read_sys_reg(vcpu, SCTLR_EL2);
-		__vcpu_write_sys_reg(vcpu, *vcpu_pc(vcpu), ELR_EL2);
+		vbar = vcpu_read_sys_reg(vcpu, VBAR_EL2);
+		sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL2);
+		vcpu_write_sys_reg(vcpu, *vcpu_pc(vcpu), ELR_EL2);
 		break;
 	default:
 		/* Don't do that */
@@ -185,7 +169,7 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
  */
 static unsigned long get_except32_cpsr(struct kvm_vcpu *vcpu, u32 mode)
 {
-	u32 sctlr = __vcpu_read_sys_reg(vcpu, SCTLR_EL1);
+	u32 sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
 	unsigned long old, new;
 
 	old = *vcpu_cpsr(vcpu);
@@ -281,7 +265,7 @@ static void enter_exception32(struct kvm_vcpu *vcpu, u32 mode, u32 vect_offset)
 {
 	unsigned long spsr = *vcpu_cpsr(vcpu);
 	bool is_thumb = (spsr & PSR_AA32_T_BIT);
-	u32 sctlr = __vcpu_read_sys_reg(vcpu, SCTLR_EL1);
+	u32 sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
 	u32 return_address;
 
 	*vcpu_cpsr(vcpu) = get_except32_cpsr(vcpu, mode);
@@ -305,7 +289,7 @@ static void enter_exception32(struct kvm_vcpu *vcpu, u32 mode, u32 vect_offset)
 	if (sctlr & (1 << 13))
 		vect_offset += 0xffff0000;
 	else /* always have security exceptions */
-		vect_offset += __vcpu_read_sys_reg(vcpu, VBAR_EL1);
+		vect_offset += vcpu_read_sys_reg(vcpu, VBAR_EL1);
 
 	*vcpu_pc(vcpu) = vect_offset;
 }

-- 
Without deviation from the norm, progress is not possible.