Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit fa731587 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf: Fix capabilities bitfield compatibility in 'struct perf_event_mmap_page' 



Solve the problems around the broken definition of perf_event_mmap_page::
cap_usr_time and cap_usr_rdpmc fields which used to overlap, partially
fixed by:

  860f085b ("perf: Fix broken union in 'struct perf_event_mmap_page'")

The problem with the fix (merged in v3.12-rc1 and not yet released
officially), noticed by Vince Weaver is that the new behavior is
not detectable by new user-space, and that due to the reuse of the
field names it's easy to mis-compile a binary if old headers are used
on a new kernel or new headers are used on an old kernel.

To solve all that make this change explicit, detectable and self-contained,
by iterating the ABI the following way:

 - Always clear bit 0, and rename it to usrpage->cap_bit0, to at least not
   confuse old user-space binaries. RDPMC will be marked as unavailable
   to old binaries but that's within the ABI, this is a capability bit.

 - Rename bit 1 to ->cap_bit0_is_deprecated and always set it to 1, so new
   libraries can reliably detect that bit 0 is deprecated and perma-zero
   without having to check the kernel version.

 - Use bits 2, 3, 4 for the newly defined, correct functionality:

	cap_user_rdpmc		: 1, /* The RDPMC instruction can be used to read counts */
	cap_user_time		: 1, /* The time_* fields are used */
	cap_user_time_zero	: 1, /* The time_zero field is used */

 - Rename all the bitfield names in perf_event.h to be different from the
   old names, to make sure it's not possible to mis-compile it
   accidentally with old assumptions.

The 'size' field can then be used in the future to add new fields and it
will act as a natural ABI version indicator as well.

Also adjust tools/perf/ userspace for the new definitions, noticed by
Adrian Hunter.

Reported-by: default avatarVince Weaver <vincent.weaver@maine.edu>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Also-Fixed-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Link: http://lkml.kernel.org/n/tip-zr03yxjrpXesOzzupszqglbv@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 73c4427c

arch/x86/kernel/cpu/perf_event.c

+5 −5
Original line number Diff line number Diff line
@@ -1883,9 +1883,9 @@ static struct pmu pmu = { 


void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)

{

	userpg->cap_usr_time = 0;

	userpg->cap_usr_time_zero = 0;

	userpg->cap_usr_rdpmc = x86_pmu.attr_rdpmc;

	userpg->cap_user_time = 0;

	userpg->cap_user_time_zero = 0;

	userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;

	userpg->pmc_width = x86_pmu.cntval_bits;



	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
@@ -1894,13 +1894,13 @@ void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now) 
	if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))

		return;



	userpg->cap_usr_time = 1;

	userpg->cap_user_time = 1;

	userpg->time_mult = this_cpu_read(cyc2ns);

	userpg->time_shift = CYC2NS_SCALE_FACTOR;

	userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;



	if (sched_clock_stable && !check_tsc_disabled()) {

		userpg->cap_usr_time_zero = 1;

		userpg->cap_user_time_zero = 1;

		userpg->time_zero = this_cpu_read(cyc2ns_offset);

	}

}

include/uapi/linux/perf_event.h

+9 −5
Original line number Diff line number Diff line
@@ -380,10 +380,13 @@ struct perf_event_mmap_page { 
	union {

		__u64	capabilities;

		struct {

			__u64	cap_usr_time		: 1,

				cap_usr_rdpmc		: 1,

				cap_usr_time_zero	: 1,

				cap_____res		: 61;

			__u64	cap_bit0		: 1, /* Always 0, deprecated, see commit 860f085b74e9 */

				cap_bit0_is_deprecated	: 1, /* Always 1, signals that bit 0 is zero */



				cap_user_rdpmc		: 1, /* The RDPMC instruction can be used to read counts */

				cap_user_time		: 1, /* The time_* fields are used */

				cap_user_time_zero	: 1, /* The time_zero field is used */

				cap_____res		: 59;

		};

	};
@@ -442,12 +445,13 @@ struct perf_event_mmap_page { 
	 *               ((rem * time_mult) >> time_shift);

	 */

	__u64	time_zero;

	__u32	size;			/* Header size up to __reserved[] fields. */



		/*

		 * Hole for extension of the self monitor capabilities

		 */



	__u64	__reserved[119];	/* align to 1k */

	__u8	__reserved[118*8+4];	/* align to 1k. */



	/*

	 * Control data for the mmap() data buffer.

kernel/events/core.c

+21 −0
Original line number Diff line number Diff line
@@ -3660,6 +3660,26 @@ static void calc_timer_values(struct perf_event *event, 
	*running = ctx_time - event->tstamp_running;

}



static void perf_event_init_userpage(struct perf_event *event)

{

	struct perf_event_mmap_page *userpg;

	struct ring_buffer *rb;



	rcu_read_lock();

	rb = rcu_dereference(event->rb);

	if (!rb)

		goto unlock;



	userpg = rb->user_page;



	/* Allow new userspace to detect that bit 0 is deprecated */

	userpg->cap_bit0_is_deprecated = 1;

	userpg->size = offsetof(struct perf_event_mmap_page, __reserved);



unlock:

	rcu_read_unlock();

}



void __weak arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)

{

}
@@ -4044,6 +4064,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma) 
	ring_buffer_attach(event, rb);

	rcu_assign_pointer(event->rb, rb);



	perf_event_init_userpage(event);

	perf_event_update_userpage(event);



unlock:

tools/perf/arch/x86/util/tsc.c

+3 −3
Original line number Diff line number Diff line
@@ -32,7 +32,7 @@ u64 tsc_to_perf_time(u64 cyc, struct perf_tsc_conversion *tc) 
int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,

			     struct perf_tsc_conversion *tc)

{

	bool cap_usr_time_zero;

	bool cap_user_time_zero;

	u32 seq;

	int i = 0;
@@ -42,7 +42,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc, 
		tc->time_mult = pc->time_mult;

		tc->time_shift = pc->time_shift;

		tc->time_zero = pc->time_zero;

		cap_usr_time_zero = pc->cap_usr_time_zero;

		cap_user_time_zero = pc->cap_user_time_zero;

		rmb();

		if (pc->lock == seq && !(seq & 1))

			break;
@@ -52,7 +52,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc, 
		}

	}



	if (!cap_usr_time_zero)

	if (!cap_user_time_zero)

		return -EOPNOTSUPP;



	return 0;