[RFC 4/6] arm64: vdso: hyperv: Map tsc page into vDSO if enabled

[Boqun Feng]

On Hyper-V, a tsc page has the data for adjusting cntvct numbers to
clocksource cycles, and that's how Hyper-V guest kernel reads the
clocksource. In order to allow userspace to read the same clocksource
directly, the tsc page has to been mapped into userspace via vDSO.

Use the framework for vDSO set-up in __vdso_init() to do this.

Note: if HYPERV_TIMER=y but the kernel is using other clocksource or
doesn't have the hyperv timer clocksource, tsc page will still be mapped
into userspace.

Signed-off-by: Boqun Feng (Microsoft) <boqun.feng@xxxxxxxxx>
---
 arch/arm64/kernel/vdso.c          | 12 ++++++++++++
 arch/arm64/kernel/vdso/vdso.lds.S | 12 +++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index b9b5ec7a3084..18a634987bdc 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -9,6 +9,7 @@
 
 #include <linux/cache.h>
 #include <linux/clocksource.h>
+#include <clocksource/hyperv_timer.h>
 #include <linux/elf.h>
 #include <linux/err.h>
 #include <linux/errno.h>
@@ -105,14 +106,22 @@ static int __vdso_init(enum arch_vdso_type arch_index)
 	struct page **vdso_code_pagelist;
 	unsigned long nr_vdso_pages;
 	unsigned long pfn;
+	struct ms_hyperv_tsc_page *tsc_page;
+	int tsc_page_idx;
 
 	if (memcmp(vdso_lookup[arch_index].vdso_code_start, "\177ELF", 4)) {
 		pr_err("vDSO is not a valid ELF object!\n");
 		return -EINVAL;
 	}
 
+	/* One vDSO data page */
 	vdso_lookup[arch_index].nr_vdso_data_pages = 1;
 
+	/* Grab the Hyper-V tsc page, if enabled, add one more page */
+	tsc_page = hv_get_tsc_page();
+	if (tsc_page)
+		tsc_page_idx = vdso_lookup[arch_index].nr_vdso_data_pages++;
+
 	vdso_lookup[arch_index].nr_vdso_code_pages = (
 			vdso_lookup[arch_index].vdso_code_end -
 			vdso_lookup[arch_index].vdso_code_start) >>
@@ -130,6 +139,9 @@ static int __vdso_init(enum arch_vdso_type arch_index)
 	/* Grab the vDSO data page. */
 	vdso_pagelist[0] = phys_to_page(__pa_symbol(vdso_data));
 
+	if (tsc_page)
+		vdso_pagelist[tsc_page_idx] = phys_to_page(__pa(tsc_page));
+
 	/* Grab the vDSO code pages. */
 	pfn = sym_to_pfn(vdso_lookup[arch_index].vdso_code_start);
 
diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
index 7ad2d3a0cd48..e40a1f5a6d30 100644
--- a/arch/arm64/kernel/vdso/vdso.lds.S
+++ b/arch/arm64/kernel/vdso/vdso.lds.S
@@ -17,7 +17,17 @@ OUTPUT_ARCH(aarch64)
 
 SECTIONS
 {
-	PROVIDE(_vdso_data = . - PAGE_SIZE);
+	/*
+	 * vdso data pages:
+	 *   vdso data (1 page)
+	 *   hv tsc page (1 page if enabled)
+	 */
+	PROVIDE(_vdso_data = _hvclock_page - PAGE_SIZE);
+#ifdef CONFIG_HYPERV_TIMER
+	PROVIDE(_hvclock_page = . - PAGE_SIZE);
+#else
+	PROVIDE(_hvclock_page = .);
+#endif
 	. = VDSO_LBASE + SIZEOF_HEADERS;
 
 	.hash		: { *(.hash) }			:text
-- 
2.24.0