[PATCH v1 1/6] KVM: arm64: Abstract the size of the HYP vectors pre-amble

[James Morse <james.morse@arm.com>]

The EL2 vector hardening feature causes KVM to generate vectors for
each type of CPU present in the system. The generated sequences already
do some of the early guest-exit work (i.e. saving registers). To avoid
duplication the generated vectors branch to the original vector just
after the preamble. This size is hard coded.

Adding new instructions to the HYP vector causes strange side effects,
which are difficult to debug as the affected code is patched in at
runtime.

Add KVM_VECTOR_PREAMBLE to tell kvm_patch_vector_branch() how big
the preamble is. The valid_vect macro can then validate this at
build time.

Signed-off-by: James Morse <james.morse@arm.com>
---
 arch/arm64/include/asm/kvm_asm.h |  6 ++++++
 arch/arm64/kvm/hyp/hyp-entry.S   | 10 +++++++++-
 arch/arm64/kvm/va_layout.c       |  7 +++----
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index ff73f5462aca..9170c43b332f 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -41,6 +41,12 @@
 	{ARM_EXCEPTION_TRAP, 		"TRAP"		},	\
 	{ARM_EXCEPTION_HYP_GONE,	"HYP_GONE"	}
 
+/*
+ * Size of the HYP vectors preamble. kvm_patch_vector_branch() generates code
+ * that jumps over this.
+ */
+#define KVM_VECTOR_PREAMBLE	4
+
 #ifndef __ASSEMBLY__
 
 #include <linux/mm.h>
diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index 2b1e686772bf..914036e6b6d7 100644
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -229,8 +229,15 @@ ENDPROC(\label)
 
 .macro valid_vect target
 	.align 7
+661:
 	stp	x0, x1, [sp, #-16]!
+662:
 	b	\target
+
+/* kvm_patch_vector_branch() generates code that jumps over the preamble. */
+.if ((662b-661b) != KVM_VECTOR_PREAMBLE)
+	.error "KVM vector preamble length mismatch"
+.endif
 .endm
 
 .macro invalid_vect target
@@ -282,7 +289,8 @@ ENDPROC(__kvm_hyp_vector)
  * movk	x0, #((addr >> 32) & 0xffff), lsl #32
  * br	x0
  *
- * Where addr = kern_hyp_va(__kvm_hyp_vector) + vector-offset + 4.
+ * Where:
+ * addr = kern_hyp_va(__kvm_hyp_vector) + vector-offset + KVM_VECTOR_PREAMBLE.
  * See kvm_patch_vector_branch for details.
  */
 alternative_cb	kvm_patch_vector_branch
diff --git a/arch/arm64/kvm/va_layout.c b/arch/arm64/kvm/va_layout.c
index c712a7376bc1..58b3a91db892 100644
--- a/arch/arm64/kvm/va_layout.c
+++ b/arch/arm64/kvm/va_layout.c
@@ -181,11 +181,10 @@ void kvm_patch_vector_branch(struct alt_instr *alt,
 	addr |= ((u64)origptr & GENMASK_ULL(10, 7));
 
 	/*
-	 * Branch to the second instruction in the vectors in order to
-	 * avoid the initial store on the stack (which we already
-	 * perform in the hardening vectors).
+	 * Branch over the preamble in order to avoid the initial store on
+	 * the stack (which we already perform in the hardening vectors).
 	 */
-	addr += AARCH64_INSN_SIZE;
+	addr += KVM_VECTOR_PREAMBLE;
 
 	/* stp x0, x1, [sp, #-16]! */
 	insn = aarch64_insn_gen_load_store_pair(AARCH64_INSN_REG_0,
-- 
2.20.1

_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm