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Subject[PATCH] x86/vdso: Give the [ph]vclock_page declarations real types
Date
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@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andy Lutomirski <luto@kernel.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
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