[PATCH v6 14/64] KVM: arm64: nv: Handle SPSR_EL2 specially

Marc Zyngier
2022-01-28T12:20:19Z

SPSR_EL2 needs special attention when running nested on ARMv8.3:

If taking an exception while running at vEL2 (actually EL1), the
HW will update the SPSR_EL1 register with the EL1 mode. We need
to track this in order to make sure that accesses to the virtual
view of SPSR_EL2 is correct.

To do so, we place an illegal value in SPSR_EL1.M, and patch it
accordingly if required when accessing it.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/kvm_emulate.h | 37 ++++++++++++++++++++++++++++
arch/arm64/kvm/sys_regs.c | 23 +++++++++++++++--
2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index cb9f123d26f3..54e8eee413eb 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -241,6 +241,43 @@ static inline bool is_hyp_ctxt(const struct kvm_vcpu *vcpu)
return __is_hyp_ctxt(&vcpu->arch.ctxt);
}

+static inline u64 __fixup_spsr_el2_write(struct kvm_cpu_context *ctxt, u64 val)
+{
+ if (!__vcpu_el2_e2h_is_set(ctxt)) {
+ /*
+ * Clear the .M field when writing SPSR to the CPU, so that we
+ * can detect when the CPU clobbered our SPSR copy during a
+ * local exception.
+ */
+ val &= ~0xc;
+ }
+
+ return val;
+}
+
+static inline u64 __fixup_spsr_el2_read(const struct kvm_cpu_context *ctxt, u64 val)
+{
+ if (__vcpu_el2_e2h_is_set(ctxt))
+ return val;
+
+ /*
+ * SPSR.M == 0 means the CPU has not touched the SPSR, so the
+ * register has still the value we saved on the last write.
+ */
+ if ((val & 0xc) == 0)
+ return ctxt_sys_reg(ctxt, SPSR_EL2);
+
+ /*
+ * Otherwise there was a "local" exception on the CPU,
+ * which from the guest's point of view was being taken from
+ * EL2 to EL2, although it actually happened to be from
+ * EL1 to EL1.
+ * So we need to fix the .M field in SPSR, to make it look
+ * like EL2, which is what the guest would expect.
+ */
+ return (val & ~0x0c) | CurrentEL_EL2;
+}
+
/*
* The layout of SPSR for an AArch32 state is different when observed from an
* AArch64 SPSR_ELx or an AArch32 SPSR_*. This function generates the AArch32
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 0927e6c345b4..ace4a54caef9 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -133,11 +133,14 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
goto memory_read;

/*
- * ELR_EL2 is special cased for now.
+ * ELR_EL2 and SPSR_EL2 are special cased for now.
*/
switch (reg) {
case ELR_EL2:
return read_sysreg_el1(SYS_ELR);
+ case SPSR_EL2:
+ val = read_sysreg_el1(SYS_SPSR);
+ return __fixup_spsr_el2_read(&vcpu->arch.ctxt, val);
}

/*
@@ -194,6 +197,10 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
case ELR_EL2:
write_sysreg_el1(val, SYS_ELR);
return;
+ case SPSR_EL2:
+ val = __fixup_spsr_el2_write(&vcpu->arch.ctxt, val);
+ write_sysreg_el1(val, SYS_SPSR);
+ return;
}

/* No EL1 counterpart? We're done here.? */
@@ -1610,6 +1617,18 @@ static bool access_sp_el1(struct kvm_vcpu *vcpu,
return true;
}

+static bool access_spsr_el2(struct kvm_vcpu *vcpu,
+ struct sys_reg_params *p,
+ const struct sys_reg_desc *r)
+{
+ if (p->is_write)
+ vcpu_write_sys_reg(vcpu, p->regval, SPSR_EL2);
+ else
+ p->regval = vcpu_read_sys_reg(vcpu, SPSR_EL2);
+
+ return true;
+}
+
/*
* Architected system registers.
* Important: Must be sorted ascending by Op0, Op1, CRn, CRm, Op2
@@ -2041,7 +2060,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
EL2_REG(VTCR_EL2, access_rw, reset_val, 0),

{ SYS_DESC(SYS_DACR32_EL2), NULL, reset_unknown, DACR32_EL2 },
- EL2_REG(SPSR_EL2, access_rw, reset_val, 0),
+ EL2_REG(SPSR_EL2, access_spsr_el2, reset_val, 0),
EL2_REG(ELR_EL2, access_rw, reset_val, 0),
{ SYS_DESC(SYS_SP_EL1), access_sp_el1},