Gradual removal of excessive barriers in pvclock reading functions
(commits 502dfeff239e8313bfbe906ca0a1a6827ac8481b,
a3eb97bd80134ba07864ca00747466c02118aca1) ended up removing too much:
although rdtsc is now orderd WRT other loads, there's no protection
against the compiler reordering the loads of ->version with the loads of
other fields.
E.g. on my system gcc-5.3.1 generates code which loads ->system_time and
->flags outside of the ->version test loop.
(Re)introduce the compiler barriers around accesses to the contents of
pvclock. While at this, make the function a bit more compact by
removing unnecessary local variables.
Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
arch/x86/include/asm/pvclock.h | 17 +++++------------
1 file changed, 5 insertions(+), 12 deletions(-)
diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index fdcc040..65c4de2 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -80,18 +80,11 @@ static __always_inline
unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
cycle_t *cycles, u8 *flags)
{
- unsigned version;
- cycle_t ret, offset;
- u8 ret_flags;
-
- version = src->version;
-
- offset = pvclock_get_nsec_offset(src);
- ret = src->system_time + offset;
- ret_flags = src->flags;
-
- *cycles = ret;
- *flags = ret_flags;
+ unsigned version = src->version;
+ barrier();
+ *cycles = src->system_time + pvclock_get_nsec_offset(src);
+ *flags = src->flags;
+ barrier();
return version;
}
--
2.5.5
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