From: Joey Gouly <joey.gouly@arm.com>
Date: Thu, 8 Feb 2024 10:54:21 +0000 (+0000)
Subject: KVM: arm64: add comments to __kern_hyp_va
X-Git-Tag: v6.9-rc1~91^2~11^2~4^2~7
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=d198e2668e247f40f68b008ce1d2656dbf2d47eb;p=thirdparty%2Fkernel%2Flinux.git

KVM: arm64: add comments to __kern_hyp_va

Document this function a little, to make it easier to understand.

The assembly comments were copied from the kern_hyp_va asm macro.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240208105422.3444159-2-joey.gouly@arm.com
[oliver: migrate a bit more detail from the asm variant]
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---

diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index e3e793d0ec304..7e7f276bcdd0b 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -56,11 +56,6 @@
 /*
  * Convert a kernel VA into a HYP VA.
  * reg: VA to be converted.
- *
- * The actual code generation takes place in kvm_update_va_mask, and
- * the instructions below are only there to reserve the space and
- * perform the register allocation (kvm_update_va_mask uses the
- * specific registers encoded in the instructions).
  */
 .macro kern_hyp_va	reg
 #ifndef __KVM_VHE_HYPERVISOR__
@@ -127,14 +122,29 @@ void kvm_apply_hyp_relocations(void);
 
 #define __hyp_pa(x) (((phys_addr_t)(x)) + hyp_physvirt_offset)
 
+/*
+ * Convert a kernel VA into a HYP VA.
+ *
+ * Can be called from hyp or non-hyp context.
+ *
+ * The actual code generation takes place in kvm_update_va_mask(), and
+ * the instructions below are only there to reserve the space and
+ * perform the register allocation (kvm_update_va_mask() uses the
+ * specific registers encoded in the instructions).
+ */
 static __always_inline unsigned long __kern_hyp_va(unsigned long v)
 {
+/*
+ * This #ifndef is an optimisation for when this is called from VHE hyp
+ * context.  When called from a VHE non-hyp context, kvm_update_va_mask() will
+ * replace the instructions with `nop`s.
+ */
 #ifndef __KVM_VHE_HYPERVISOR__
-	asm volatile(ALTERNATIVE_CB("and %0, %0, #1\n"
-				    "ror %0, %0, #1\n"
-				    "add %0, %0, #0\n"
-				    "add %0, %0, #0, lsl 12\n"
-				    "ror %0, %0, #63\n",
+	asm volatile(ALTERNATIVE_CB("and %0, %0, #1\n"         /* mask with va_mask */
+				    "ror %0, %0, #1\n"         /* rotate to the first tag bit */
+				    "add %0, %0, #0\n"         /* insert the low 12 bits of the tag */
+				    "add %0, %0, #0, lsl 12\n" /* insert the top 12 bits of the tag */
+				    "ror %0, %0, #63\n",       /* rotate back */
 				    ARM64_ALWAYS_SYSTEM,
 				    kvm_update_va_mask)
 		     : "+r" (v));