[PATCH 10/14] KVM: arm64: nv: Handle mapping of VNCR_EL2 at EL2

Marc Zyngier
2025-02-15T15:02:04Z

Now that we can handle faults triggered through VNCR_EL2, we need
to map the corresponding page at EL2. But where, you'll ask?

Since each CPU in the system can run a vcpu, we need a per-CPU
mapping. For that, we carve a NR_CPUS range in the fixmap, giving
us a per-CPU va at which to map the guest's VNCR's page.

The mapping occurs both on vcpu load and on the back of a fault,
both generating a request that will take care of the mapping.
That mapping will also get dropped on vcpu put.

Yes, this is a bit heavy handed, but it is simple. Eventually,
we may want to have a per-VM, per-CPU mapping, which would avoid
all the TLBI overhead.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/fixmap.h | 6 ++
arch/arm64/include/asm/kvm_host.h | 1 +
arch/arm64/include/asm/kvm_nested.h | 7 +++
arch/arm64/kvm/nested.c | 98 ++++++++++++++++++++++++++---
4 files changed, 103 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
index 87e307804b99c..635a43c4ec85b 100644
--- a/arch/arm64/include/asm/fixmap.h
+++ b/arch/arm64/include/asm/fixmap.h
@@ -48,6 +48,12 @@ enum fixed_addresses {
FIX_EARLYCON_MEM_BASE,
FIX_TEXT_POKE0,

+#ifdef CONFIG_KVM
+ /* One slot per CPU, mapping the guest's VNCR page at EL2. */
+ FIX_VNCR_END,
+ FIX_VNCR = FIX_VNCR_END + NR_CPUS,
+#endif
+
#ifdef CONFIG_ACPI_APEI_GHES
/* Used for GHES mapping from assorted contexts */
FIX_APEI_GHES_IRQ,
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 3cccf6fca4dfa..0e95f4adb72ba 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -623,6 +623,7 @@ struct kvm_host_data {
#define KVM_HOST_DATA_FLAG_TRBE_ENABLED 4
#define KVM_HOST_DATA_FLAG_EL1_TRACING_CONFIGURED 5
#define KVM_HOST_DATA_FLAG_VCPU_IN_HYP_CONTEXT 6
+#define KVM_HOST_DATA_FLAG_L1_VNCR_MAPPED 7
unsigned long flags;

struct kvm_cpu_context host_ctxt;
diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
index 53ff314b9ecd1..284bedee742ec 100644
--- a/arch/arm64/include/asm/kvm_nested.h
+++ b/arch/arm64/include/asm/kvm_nested.h
@@ -336,4 +336,11 @@ int __kvm_translate_va(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
int kvm_vcpu_allocate_vncr_tlb(struct kvm_vcpu *vcpu);
int kvm_handle_vncr_abort(struct kvm_vcpu *vcpu);

+#define vncr_fixmap(c) \
+ ({ \
+ u32 __c = (c); \
+ BUG_ON(__c >= NR_CPUS); \
+ (FIX_VNCR - __c); \
+ })
+
#endif /* __ARM64_KVM_NESTED_H */
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 2c4991a84e003..51e1a3eba996a 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -8,6 +8,7 @@
#include <linux/kvm.h>
#include <linux/kvm_host.h>

+#include <asm/fixmap.h>
#include <asm/kvm_arm.h>
#include <asm/kvm_emulate.h>
#include <asm/kvm_mmu.h>
@@ -702,23 +703,35 @@ void kvm_init_nested_s2_mmu(struct kvm_s2_mmu *mmu)
void kvm_vcpu_load_hw_mmu(struct kvm_vcpu *vcpu)
{
/*
- * The vCPU kept its reference on the MMU after the last put, keep
- * rolling with it.
+ * If the vCPU kept its reference on the MMU after the last put,
+ * keep rolling with it.
*/
- if (vcpu->arch.hw_mmu)
- return;
-
if (is_hyp_ctxt(vcpu)) {
- vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
+ if (!vcpu->arch.hw_mmu)
+ vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
} else {
- write_lock(&vcpu->kvm->mmu_lock);
- vcpu->arch.hw_mmu = get_s2_mmu_nested(vcpu);
- write_unlock(&vcpu->kvm->mmu_lock);
+ if (!vcpu->arch.hw_mmu) {
+ scoped_guard(write_lock, &vcpu->kvm->mmu_lock)
+ vcpu->arch.hw_mmu = get_s2_mmu_nested(vcpu);
+ }
+
+ if (__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_NV)
+ kvm_make_request(KVM_REQ_MAP_L1_VNCR_EL2, vcpu);
}
}

void kvm_vcpu_put_hw_mmu(struct kvm_vcpu *vcpu)
{
+ /* Unconditionally drop the VNCR mapping if we have one */
+ if (host_data_test_flag(L1_VNCR_MAPPED)) {
+ BUG_ON(vcpu->arch.vncr_tlb->cpu != smp_processor_id());
+ BUG_ON(is_hyp_ctxt(vcpu));
+
+ clear_fixmap(vncr_fixmap(vcpu->arch.vncr_tlb->cpu));
+ vcpu->arch.vncr_tlb->cpu = -1;
+ host_data_clear_flag(L1_VNCR_MAPPED);
+ }
+
/*
* Keep a reference on the associated stage-2 MMU if the vCPU is
* scheduling out and not in WFI emulation, suggesting it is likely to
@@ -1039,6 +1052,70 @@ int kvm_handle_vncr_abort(struct kvm_vcpu *vcpu)
return 1;
}

+static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
+{
+ struct vncr_tlb *vt = vcpu->arch.vncr_tlb;
+ pgprot_t prot;
+
+ guard(preempt)();
+ guard(read_lock)(&vcpu->kvm->mmu_lock);
+
+ /*
+ * The request to map VNCR may have raced against some other
+ * event, such as an interrupt, and may not be valid anymore.
+ */
+ if (is_hyp_ctxt(vcpu))
+ return;
+
+ /*
+ * Check that the pseudo-TLB is valid and that VNCR_EL2 still
+ * contains the expected value. If it doesn't, we simply bail out
+ * without a mapping -- a transformed MSR/MRS will generate the
+ * fault and allows us to populate the pseudo-TLB.
+ */
+ if (!vt->valid)
+ return;
+
+ if (read_vncr_el2(vcpu) != vt->gva)
+ return;
+
+ if (vt->wr.nG) {
+ u64 tcr = vcpu_read_sys_reg(vcpu, TCR_EL2);
+ u64 ttbr = ((tcr & TCR_A1) ?
+ vcpu_read_sys_reg(vcpu, TTBR1_EL2) :
+ vcpu_read_sys_reg(vcpu, TTBR0_EL2));
+ u16 asid;
+
+ asid = FIELD_GET(TTBR_ASID_MASK, ttbr);
+ if (!kvm_has_feat_enum(vcpu->kvm, ID_AA64MMFR0_EL1, ASIDBITS, 16) ||
+ !(tcr & TCR_ASID16))
+ asid &= GENMASK(7, 0);
+
+ if (asid != vt->wr.asid)
+ return;
+ }
+
+ vt->cpu = smp_processor_id();
+
+ if (vt->wr.pw && vt->wr.pr)
+ prot = PAGE_KERNEL;
+ else if (vt->wr.pr)
+ prot = PAGE_KERNEL_RO;
+ else
+ prot = PAGE_NONE;
+
+ /*
+ * We can't map write-only (or no permission at all) in the kernel,
+ * but the guest can do it if using POE, so we'll have to turn a
+ * translation fault into a permission fault at runtime.
+ * FIXME: WO doesn't work at all, need POE support in the kernel.
+ */
+ if (pgprot_val(prot) != pgprot_val(PAGE_NONE)) {
+ __set_fixmap(vncr_fixmap(vt->cpu), vt->hpa, prot);
+ host_data_set_flag(L1_VNCR_MAPPED);
+ }
+}
+
/*
* Our emulated CPU doesn't support all the possible features. For the
* sake of simplicity (and probably mental sanity), wipe out a number
@@ -1551,4 +1628,7 @@ void check_nested_vcpu_requests(struct kvm_vcpu *vcpu)
}
write_unlock(&vcpu->kvm->mmu_lock);
}
+
+ if (kvm_check_request(KVM_REQ_MAP_L1_VNCR_EL2, vcpu))
+ kvm_map_l1_vncr(vcpu);
}
--
2.39.2