[PATCH v2 11/15] KVM: arm64: nv: Honor guest hypervisor's FP/SVE traps in CPTR_EL2

Oliver Upton
2024-06-13T20:18:51Z

Start folding the guest hypervisor's FP/SVE traps into the value
programmed in hardware. Note that as of writing this is dead code, since
KVM does a full put() / load() for every nested exception boundary which
saves + flushes the FP/SVE state.

However, this will become useful when we can keep the guest's FP/SVE
state alive across a nested exception boundary and the host no longer
needs to conservatively program traps.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/kvm/hyp/vhe/switch.c | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)

diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 925de4b4efd2..b0b1935a3626 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -67,6 +67,8 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)

static void __activate_cptr_traps(struct kvm_vcpu *vcpu)
{
+ u64 cptr;
+
/*
* With VHE (HCR.E2H == 1), accesses to CPACR_EL1 are routed to
* CPTR_EL2. In general, CPACR_EL1 has the same layout as CPTR_EL2,
@@ -85,6 +87,35 @@ static void __activate_cptr_traps(struct kvm_vcpu *vcpu)
__activate_traps_fpsimd32(vcpu);
}

+ /*
+ * Layer the guest hypervisor's trap configuration on top of our own if
+ * we're in a nested context.
+ */
+ if (!vcpu_has_nv(vcpu) || is_hyp_ctxt(vcpu))
+ goto write;
+
+ cptr = vcpu_sanitised_cptr_el2(vcpu);
+
+ /*
+ * Pay attention, there's some interesting detail here.
+ *
+ * The CPTR_EL2.xEN fields are 2 bits wide, although there are only two
+ * meaningful trap states when HCR_EL2.TGE = 0 (running a nested guest):
+ *
+ * - CPTR_EL2.xEN = x0, traps are enabled
+ * - CPTR_EL2.xEN = x1, traps are disabled
+ *
+ * In other words, bit[0] determines if guest accesses trap or not. In
+ * the interest of simplicity, clear the entire field if the guest
+ * hypervisor has traps enabled to dispel any illusion of something more
+ * complicated taking place.
+ */
+ if (!(SYS_FIELD_GET(CPACR_ELx, FPEN, cptr) & BIT(0)))
+ val &= ~CPACR_ELx_FPEN;
+ if (!(SYS_FIELD_GET(CPACR_ELx, ZEN, cptr) & BIT(0)))
+ val &= ~CPACR_ELx_ZEN;
+
+write:
write_sysreg(val, cpacr_el1);
}

--
2.45.2.627.g7a2c4fd464-goog