[PATCH 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 to match 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:

Layout with "hsr_high" added:

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

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

Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---
I'm happy to drop the patch if this change is premature; I made the change
now, instead of waiting until there's a use case for the upper 32 bits, so
userspace has had the time to pick up the updated struct definition by the
time KVM will start making use of the new field.

 arch/arm64/include/uapi/asm/kvm.h | 1 +
 arch/arm64/kvm/handle_exit.c      | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index c1b6ddc02d2f..f51f9e413cb8 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -141,6 +141,7 @@ struct kvm_guest_debug_arch {
 
 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..2ccf4e6372c2 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -121,6 +121,7 @@ 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);
 
 	if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
 		run->debug.arch.far = vcpu->arch.fault.far_el2;
-- 
2.25.1