arm64: hyperv: Fix build breakage for non-ARM64 architectures

Commit [arm64: hyperv: Enable Hyper-V synthetic clocks/timers]
introduced changes to enable Hyper-V synthetic clocks and timers on
ARM64. However, these upstream changes led to build breakage on ARM64
architectures due to the use of the '__bss_decrypted' section, which is
not available on ARM64.

To address this issue, this commit adds conditional compilation to use
'__bss_decrypted' only on non-ARM64 architectures. The introduced
'#ifdef CONFIG_ARM64' ensures that the code involving '__bss_decrypted'
is not included when building for ARM64, preventing build failures.

Signed-off-by: Allen Pais <apais@microsoft.com>
[ Fix merge conflicts with fe11f97 ("hyperv-tlfs: Change prefix of
generic HV_REGISTER_* MSRs to HV_MSR_*"), reword commit message ]
Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>

Author: allenpais

arch/arm64/include/asm/hyperv_timer.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM64_HYPERV_TIMER_H
+#define _ASM_ARM64_HYPERV_TIMER_H
+
+#include <asm/mshyperv.h>
+
+#endif

drivers/clocksource/hyperv_timer.c

@@ -373,11 +373,15 @@ static __always_inline u64 read_hv_clock_msr(void)
 	 * is set to 0 when the partition is created and is incremented in 100
 	 * nanosecond units.
 	 *
-	 * Use hv_raw_get_msr() because this function is used from
-	 * noinstr. Notable; while HV_MSR_TIME_REF_COUNT is a synthetic
-	 * register it doesn't need the GHCB path.
+	 * Use hv_raw_get_msr() on x86 because this function is used from noinstr
+	 * on x86. Notable; while HV_MSR_TIME_REF_COUNT is a synthetic register
+	 * it doesn't need the GHCB path.
 	 */
+#ifdef CONFIG_ARM64
+	return hv_get_msr(HV_MSR_TIME_REF_COUNT);
+#else
 	return hv_raw_get_msr(HV_MSR_TIME_REF_COUNT);
+#endif
 }
 
 /*
@@ -391,7 +395,12 @@ static __always_inline u64 read_hv_clock_msr(void)
 static union {
 	struct ms_hyperv_tsc_page page;
 	u8 reserved[PAGE_SIZE];
-} tsc_pg __bss_decrypted __aligned(PAGE_SIZE);
+} tsc_pg
+#ifdef CONFIG_ARM64
+    __aligned(PAGE_SIZE);
+#else
+    __bss_decrypted __aligned(PAGE_SIZE);
+#endif
 
 static struct ms_hyperv_tsc_page *tsc_page = &tsc_pg.page;
 static unsigned long tsc_pfn;