linux-kernel - [PATCH] x86/vdso: Give the [ph]vclock

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Date:   Sat, 22 Jun 2019 15:08:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] x86/vdso: Give the [ph]vclock_page declarations real types

Clean up the vDSO code a bit by giving pvclock_page and hvclock_page
their actual types instead of u8[PAGE_SIZE].  This shouldn't
materially affect the generated code.

Heavily based on a patch from Linus.

Cc: Borislav Petkov <bp@...en8.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/vdso/vclock_gettime.c | 36 ++++++++++++++++++----------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 4aed41f638bb..907efc5015ec 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -28,13 +28,33 @@ extern int __vdso_clock_gettime(clockid_t clock, struct timespec *ts);
 extern int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz);
 extern time_t __vdso_time(time_t *t);
 
+/*
+ * Declare the memory-mapped vclock data pages.  These come from hypervisors.
+ * If we ever reintroduce something like direct access to an MMIO clock like
+ * the HPET again, it will go here as well.
+ *
+ * A load from any of these pages will segfault if the clock in question is
+ * disabled, so appropriate compiler barriers and checks need to be used
+ * to prevent stray loads.
+ *
+ * These declarations MUST NOT be const.  The compiler will assume that
+ * an extern const variable has genuinely constant contents, and the
+ * resulting code won't work, since the whole point is that these pages
+ * change over time, possibly while we're accessing them.
+ */
+
 #ifdef CONFIG_PARAVIRT_CLOCK
-extern u8 pvclock_page[PAGE_SIZE]
+/*
+ * This is the vCPU 0 pvclock page.  We only use pvclock from the vDSO
+ * if the hypervisor tells us that all vCPUs can get valid data from the
+ * vCPU 0 page.
+ */
+extern struct pvclock_vsyscall_time_info pvclock_page
 	__attribute__((visibility("hidden")));
 #endif
 
 #ifdef CONFIG_HYPERV_TSCPAGE
-extern u8 hvclock_page[PAGE_SIZE]
+extern struct ms_hyperv_tsc_page hvclock_page
 	__attribute__((visibility("hidden")));
 #endif
 
@@ -69,14 +89,9 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 #endif
 
 #ifdef CONFIG_PARAVIRT_CLOCK
-static notrace const struct pvclock_vsyscall_time_info *get_pvti0(void)
-{
-	return (const struct pvclock_vsyscall_time_info *)&pvclock_page;
-}
-
 static notrace u64 vread_pvclock(void)
 {
-	const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
+	const struct pvclock_vcpu_time_info *pvti = &pvclock_page.pvti;
 	u32 version;
 	u64 ret;
 
@@ -117,10 +132,7 @@ static notrace u64 vread_pvclock(void)
 #ifdef CONFIG_HYPERV_TSCPAGE
 static notrace u64 vread_hvclock(void)
 {
-	const struct ms_hyperv_tsc_page *tsc_pg =
-		(const struct ms_hyperv_tsc_page *)&hvclock_page;
-
-	return hv_read_tsc_page(tsc_pg);
+	return hv_read_tsc_page(&hvclock_page);
 }
 #endif
 
-- 
2.21.0