[PATCH v2 5/5] KVM: arm64: uapi: Add kvm_debug_exit_arch.hsr_high

[Alexandru Elisei]

When userspace is debugging a VM, the kvm_debug_exit_arch part of the
kvm_run struct contains arm64 specific debug information: the ESR_EL2
value, encoded in the field "hsr", and the address of the instruction
that caused the exception, encoded in the field "far".

Linux has moved to treating ESR_EL2 as a 64-bit register, but unfortunately
kvm_debug_exit_arch.hsr cannot be changed because that would change the
memory layout of the struct on big endian machines:

Current layout:			| Layout with "hsr" extended to 64 bits:
				|
offset 0: ESR_EL2[31:0] (hsr)   | offset 0: ESR_EL2[61:32] (hsr[61:32])
offset 4: padding		| offset 4: ESR_EL2[31:0]  (hsr[31:0])
offset 8: FAR_EL2[61:0] (far)	| offset 8: FAR_EL2[61:0]  (far)

which breaks existing code.

The padding is inserted by the compiler because the "far" field must be
aligned to 8 bytes (each field must be naturally aligned - aapcs64 [1],
page 18), and the struct itself must be aligned to 8 bytes (the struct must
be aligned to the maximum alignment of its fields - aapcs64, page 18),
which means that "hsr" must be aligned to 8 bytes as it is the first field
in the struct.

To avoid changing the struct size and layout for the existing fields, add a
new field, "hsr_high", which replaces the existing padding. "hsr_high" will
be used to hold the ESR_EL2[61:32] bits of the register. The memory layout,
both on big and little endian machine, becomes:

offset 0: ESR_EL2[31:0]  (hsr)
offset 4: ESR_EL2[61:32] (hsr_high)
offset 8: FAR_EL2[61:0]  (far)

The padding that the compiler inserts for the current struct layout is
unitialized. To prevent an updated userspace running on an old kernel
mistaking the padding for a valid "hsr_high" value, add a new flag,
KVM_DEBUG_ARCH_HSR_HIGH_VALID, to kvm_run->flags to let userspace know that
"hsr_high" holds a valid ESR_EL2[61:32] value.

[1] https://github.com/ARM-software/abi-aa/releases/download/2021Q3/aapcs64.pdf

Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---
 arch/arm64/include/uapi/asm/kvm.h | 2 ++
 arch/arm64/kvm/handle_exit.c      | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index c1b6ddc02d2f..695324b6f92e 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -139,8 +139,10 @@ struct kvm_guest_debug_arch {
 	__u64 dbg_wvr[KVM_ARM_MAX_DBG_REGS];
 };
 
+#define KVM_DEBUG_ARCH_HSR_HIGH_VALID	1
 struct kvm_debug_exit_arch {
 	__u32 hsr;
+	__u32 hsr_high;	/* ESR_EL2[61:32] */
 	__u64 far;	/* used for watchpoints */
 };
 
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 93d92130d36c..fd5b6773e3a2 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -121,6 +121,8 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
 
 	run->exit_reason = KVM_EXIT_DEBUG;
 	run->debug.arch.hsr = lower_32_bits(esr);
+	run->debug.arch.hsr_high = upper_32_bits(esr);
+	run->flags |= KVM_DEBUG_ARCH_HSR_HIGH_VALID;
 
 	if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
 		run->debug.arch.far = vcpu->arch.fault.far_el2;
-- 
2.36.0