[RFC PATCH v3 07/11] arm64: compat: Add a 32-bit vDSO
Nathan Lynch
nathan_lynch at mentor.com
Thu Dec 8 09:02:20 PST 2016
Kevin Brodsky <kevin.brodsky at arm.com> writes:
> On 07/12/16 18:51, Nathan Lynch wrote:
>> Kevin Brodsky <kevin.brodsky at arm.com> writes:
>>> diff --git a/arch/arm64/kernel/vdso32/vgettimeofday.c b/arch/arm64/kernel/vdso32/vgettimeofday.c
>>> new file mode 100644
>>> index 000000000000..53c3d1f82b26
>>> --- /dev/null
>>> +++ b/arch/arm64/kernel/vdso32/vgettimeofday.c
>> As I said at LPC last month, I'm not excited to have arch/arm's
>> vgettimeofday.c copied into arch/arm64 and tweaked; I'd rather see this
>> part of the implementation shared between arch/arm and arch/arm64
>> somehow, even if there's not an elegant way to do so.
>>
>> The situation which must be avoided is one where the arch/arm64 compat
>> VDSO incompatibly diverges from the arch/arm VDSO. That becomes much
>> less likely if there's only one copy of the userspace-exposed code to
>> maintain.
>
> I still agree this is very suboptimal. However, I also think this is by far the most
> straightforward solution, and I would like to stick to it *as a first step*. If you
> diff this "tweaked" vgettimeofday.c with the original one, you'll see that it is
> really not obvious to share even parts of vgettimeofday.c in the current state of arm
> and arm64.
After adapting your get_vdso_data() to arch/arm/vdso, I come up with the
differences below, which seems surmountable mostly with the addition of
appropriate accessor functions for the data page, or changing the field
names to match. So I can't say I'm persuaded.
I'm happy to review and facilitate changes to code in arch/arm/vdso to
make it possible to share it with arm64 for its compat VDSO.
vgettimeofday.c | 84 ++++++++++++++++++++++++--------------------------------
1 file changed, 37 insertions(+), 47 deletions(-)
--- arch/arm/vdso/vgettimeofday.c 2016-12-07 11:24:35.043856904 -0600
+++ kb-vgettimeofday.c 2016-12-08 10:02:14.031896779 -0600
@@ -15,20 +15,23 @@
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
+#include <linux/clocksource.h>
#include <linux/compiler.h>
-#include <linux/hrtimer.h>
#include <linux/time.h>
-#include <asm/arch_timer.h>
-#include <asm/barrier.h>
-#include <asm/bug.h>
-#include <asm/page.h>
#include <asm/unistd.h>
#include <asm/vdso_datapage.h>
-#ifndef CONFIG_AEABI
-#error This code depends on AEABI system call conventions
-#endif
+#include "aarch32-barrier.h"
+/*
+ * We use the hidden visibility to prevent the compiler from generating a GOT
+ * relocation. Not only is going through a GOT useless (the entry couldn't and
+ * musn't be overridden by another library), it does not even work: the linker
+ * cannot generate an absolute address to the data page.
+ *
+ * With the hidden visibility, the compiler simply generates a PC-relative
+ * relocation (R_ARM_REL32), and this is what we need.
+ */
extern const struct vdso_data _vdso_data __attribute__((visibility("hidden")));
static inline const struct vdso_data *get_vdso_data(void)
@@ -52,13 +55,11 @@
return ret;
}
-#define __get_datapage() get_vdso_data()
-
static notrace u32 __vdso_read_begin(const struct vdso_data *vdata)
{
u32 seq;
repeat:
- seq = ACCESS_ONCE(vdata->seq_count);
+ seq = ACCESS_ONCE(vdata->tb_seq_count);
if (seq & 1) {
cpu_relax();
goto repeat;
@@ -72,26 +73,30 @@
seq = __vdso_read_begin(vdata);
- smp_rmb(); /* Pairs with smp_wmb in vdso_write_end */
+ aarch32_smp_rmb();
return seq;
}
static notrace int vdso_read_retry(const struct vdso_data *vdata, u32 start)
{
- smp_rmb(); /* Pairs with smp_wmb in vdso_write_begin */
- return vdata->seq_count != start;
+ aarch32_smp_rmb();
+ return vdata->tb_seq_count != start;
}
+/*
+ * Note: only AEABI is supported by the compat layer, we can assume AEABI
+ * syscall conventions are used.
+ */
static notrace long clock_gettime_fallback(clockid_t _clkid,
struct timespec *_ts)
{
register struct timespec *ts asm("r1") = _ts;
register clockid_t clkid asm("r0") = _clkid;
register long ret asm ("r0");
- register long nr asm("r7") = __NR_clock_gettime;
+ register long nr asm("r7") = __NR_compat_clock_gettime;
asm volatile(
- " swi #0\n"
+ " svc #0\n"
: "=r" (ret)
: "r" (clkid), "r" (ts), "r" (nr)
: "memory");
@@ -138,25 +143,27 @@
return 0;
}
-#ifdef CONFIG_ARM_ARCH_TIMER
-
static notrace u64 get_ns(const struct vdso_data *vdata)
{
u64 cycle_delta;
u64 cycle_now;
u64 nsec;
- cycle_now = arch_counter_get_cntvct();
+ /* AArch32 implementation of arch_counter_get_cntvct() */
+ isb();
+ asm volatile("mrrc p15, 1, %Q0, %R0, c14" : "=r" (cycle_now));
- cycle_delta = (cycle_now - vdata->cs_cycle_last) & vdata->cs_mask;
+ /* The virtual counter provides 56 significant bits. */
+ cycle_delta = (cycle_now - vdata->cs_cycle_last) & CLOCKSOURCE_MASK(56);
- nsec = (cycle_delta * vdata->cs_mult) + vdata->xtime_clock_snsec;
+ nsec = (cycle_delta * vdata->cs_mono_mult) + vdata->xtime_clock_nsec;
nsec >>= vdata->cs_shift;
return nsec;
}
-static notrace int do_realtime(struct timespec *ts, const struct vdso_data *vdata)
+static notrace int do_realtime(struct timespec *ts,
+ const struct vdso_data *vdata)
{
u64 nsecs;
u32 seq;
@@ -164,7 +171,7 @@
do {
seq = vdso_read_begin(vdata);
- if (!vdata->tk_is_cntvct)
+ if (vdata->use_syscall)
return -1;
ts->tv_sec = vdata->xtime_clock_sec;
@@ -178,7 +185,8 @@
return 0;
}
-static notrace int do_monotonic(struct timespec *ts, const struct vdso_data *vdata)
+static notrace int do_monotonic(struct timespec *ts,
+ const struct vdso_data *vdata)
{
struct timespec tomono;
u64 nsecs;
@@ -187,7 +195,7 @@
do {
seq = vdso_read_begin(vdata);
- if (!vdata->tk_is_cntvct)
+ if (vdata->use_syscall)
return -1;
ts->tv_sec = vdata->xtime_clock_sec;
@@ -205,27 +213,11 @@
return 0;
}
-#else /* CONFIG_ARM_ARCH_TIMER */
-
-static notrace int do_realtime(struct timespec *ts, const struct vdso_data *vdata)
-{
- return -1;
-}
-
-static notrace int do_monotonic(struct timespec *ts, const struct vdso_data *vdata)
-{
- return -1;
-}
-
-#endif /* CONFIG_ARM_ARCH_TIMER */
-
notrace int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts)
{
- const struct vdso_data *vdata;
+ const struct vdso_data *vdata = get_vdso_data();
int ret = -1;
- vdata = __get_datapage();
-
switch (clkid) {
case CLOCK_REALTIME_COARSE:
ret = do_realtime_coarse(ts, vdata);
@@ -255,10 +247,10 @@
register struct timezone *tz asm("r1") = _tz;
register struct timeval *tv asm("r0") = _tv;
register long ret asm ("r0");
- register long nr asm("r7") = __NR_gettimeofday;
+ register long nr asm("r7") = __NR_compat_gettimeofday;
asm volatile(
- " swi #0\n"
+ " svc #0\n"
: "=r" (ret)
: "r" (tv), "r" (tz), "r" (nr)
: "memory");
@@ -269,11 +261,9 @@
notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
{
struct timespec ts;
- const struct vdso_data *vdata;
+ const struct vdso_data *vdata = get_vdso_data();
int ret;
- vdata = __get_datapage();
-
ret = do_realtime(&ts, vdata);
if (ret)
return gettimeofday_fallback(tv, tz);
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