Subject: Re: [PATCH 6/8] powerpc/perf: add support for the hv gpci (get performance counter info) interface

From: Cody P Schafer <[email-masked]>
Date: Mon, 03 Feb 2014 13:13:09 -0800

On 01/31/2014 09:58 PM, Michael Ellerman wrote:
> On Thu, 2014-16-01 at 23:53:52 UTC, Cody P Schafer wrote:
>> This provides a basic link between perf and hv_gpci. Notably, it does
>> not yet support transactions and does not list any events (they can
>> still be manually composed).
>
> What are the plans for listing?

I'm looking at extending the sysfs api for listing perf events. We can't 
use the existing one as it doesn't let us parametrize the events by 
anything, meaning we'd need to list an essentially duplicate event for 
each cpu/core/chip and lpar (guest). The duplication in cpu/core/chip 
comes from not using the typical cpu parameter to perf_event_open() 
(which we don't do because it wouldn't make sense when the guest 
monitored isn't us).

> The manual compose is nice but pretty hairy to use in practice I would think.
>
>> diff --git a/arch/powerpc/perf/hv-gpci.c b/arch/powerpc/perf/hv-gpci.c
>> new file mode 100644
>> index 0000000..31d9d59
>> --- /dev/null
>> +++ b/arch/powerpc/perf/hv-gpci.c
>> @@ -0,0 +1,235 @@
>> +/*
>> + * Hypervisor supplied "gpci" ("get performance counter info") performance
>> + * counter support
>> + *
>> + * Author: Cody P Schafer <[email-masked]>
>> + * Copyright 2014 IBM Corporation.
>> + *
>> + * This program is free software; you can redistribute it and/or
>> + * modify it under the terms of the GNU General Public License
>> + * as published by the Free Software Foundation; either version
>> + * 2 of the License, or (at your option) any later version.
>> + */
>> +#define pr_fmt(fmt) "hv-gpci: " fmt
>> +
>> +#include <linux/module.h>
>> +#include <linux/perf_event.h>
>> +#include <asm/firmware.h>
>> +#include <asm/hvcall.h>
>> +#include <asm/hv_gpci.h>
>> +#include <asm/io.h>
>
> Needed?
>

asm/io.h is for virt_to_phys(). And yes, I need it.

>> +/* See arch/powerpc/include/asm/hv_gpci.h for details on the hcall interface */
>> +
>> +PMU_RANGE_ATTR(request, config, 0, 31); /* u32 */
>> +PMU_RANGE_ATTR(starting_index, config, 32, 63); /* u32 */
>> +PMU_RANGE_ATTR(secondary_index, config1, 0, 15); /* u16 */
>> +PMU_RANGE_ATTR(counter_info_version, config1, 16, 23); /* u8 */
>> +PMU_RANGE_ATTR(length, config1, 24, 31); /* u8, bytes of data (1-8) */
>> +PMU_RANGE_ATTR(offset, config1, 32, 63); /* u32, byte offset */
>> +
>> +static struct attribute *format_attr[] = {
>> +	&format_attr_request.attr,
>> +	&format_attr_starting_index.attr,
>> +	&format_attr_secondary_index.attr,
>> +	&format_attr_counter_info_version.attr,
>> +
> Lonley blank line.

Which was seperating "real" attributes from those provided by the kernel 
(gpci doesn't know about "offset" and "length"). I'll remove.

>> +	&format_attr_offset.attr,
>> +	&format_attr_length.attr,
>> +	NULL,
>> +};
>> +
>> +static struct attribute_group format_group = {
>> +	.name = "format",
>> +	.attrs = format_attr,
>> +};
>> +
>> +static const struct attribute_group *attr_groups[] = {
>> +	&format_group,
>> +	NULL,
>> +};
>> +
>> +static unsigned long single_gpci_request(u32 req, u32 starting_index,
>> +		u16 secondary_index, u8 version_in, u32 offset, u8 length,
>> +		u64 *value)
>
> Passing the event and extracting the values in here would be neater IMHO.
>

Well, I'll at least add a wrapper that does that. The idea here was to 
separate the perf specific logic from the gpci specific logic. And I'll 
end up taking advantage of that separation when doing the interface 
probing (mentioned way down near the end of this email).

>> +{
>> +	unsigned long ret;
>> +	size_t i;
>> +	u64 count;
>> +
>> +	struct {
>> +		struct hv_get_perf_counter_info_params params;
>> +		union {
>> +			union h_gpci_cvs data;
>> +			uint8_t bytes[sizeof(union h_gpci_cvs)];
>> +		};
>> +	} arg = {
>> +		.params = {
>> +			.counter_request = cpu_to_be32(req),
>> +			.starting_index = cpu_to_be32(starting_index),
>> +			.secondary_index = cpu_to_be16(secondary_index),
>> +			.counter_info_version_in = version_in,
>> +		}
>> +	};
>> +
>> +	ret = plpar_hcall_norets(H_GET_PERF_COUNTER_INFO,
>> +			virt_to_phys(&arg), sizeof(arg));
>> +	if (ret) {
>> +		pr_devel("hcall failed: 0x%lx\n", ret);
>> +		return ret;
>> +	}
>> +
>> +	/*
>> +	 * we verify offset and length are within the zeroed buffer at event
>> +	 * init.
>> +	 */
>> +	count = 0;
>> +	for (i = offset; i < offset + length; i++)
>> +		count |= arg.bytes[i] << (i - offset);
>> +
>> +	*value = count;
>> +	return ret;
>> +}
>> +
>> +static u64 h_gpci_get_value(struct perf_event *event)
>> +{
>> +	u64 count;
>> +	unsigned long ret = single_gpci_request(event_get_request(event),
>> +					event_get_starting_index(event),
>> +					event_get_secondary_index(event),
>> +					event_get_counter_info_version(event),
>> +					event_get_offset(event),
>> +					event_get_length(event),
>> +					&count);
>> +	if (ret)
>> +		return 0;
>> +	return count;
>> +}
>> +
>> +static void h_gpci_event_update(struct perf_event *event)
>> +{
>> +	s64 prev;
>> +	u64 now = h_gpci_get_value(event);
>> +	prev = local64_xchg(&event->hw.prev_count, now);
>> +	local64_add(now - prev, &event->count);
>> +}
>> +
>> +static void h_gpci_event_start(struct perf_event *event, int flags)
>> +{
>> +	local64_set(&event->hw.prev_count, h_gpci_get_value(event));
>> +	perf_swevent_start_hrtimer(event);
>> +}
>> +
>> +static void h_gpci_event_stop(struct perf_event *event, int flags)
>> +{
>> +	perf_swevent_cancel_hrtimer(event);
>> +	h_gpci_event_update(event);
>> +}
>> +
>> +static int h_gpci_event_add(struct perf_event *event, int flags)
>> +{
>> +	if (flags & PERF_EF_START)
>> +		h_gpci_event_start(event, flags);
>> +
>> +	return 0;
>> +}
>> +
>> +static void h_gpci_event_del(struct perf_event *event, int flags)
>> +{
>> +	h_gpci_event_stop(event, flags);
>> +}
>
> Can just hook del directly no?

Yep, good point.

>
>> +static void h_gpci_event_read(struct perf_event *event)
>> +{
>> +	h_gpci_event_update(event);
>> +}
>
> Ditto.

Yep, agreed.

>
>> +static int h_gpci_event_init(struct perf_event *event)
>> +{
>> +	u64 count;
>> +	u8 length;
>> +
>> +	/* Not our event */
>> +	if (event->attr.type != event->pmu->type)
>> +		return -ENOENT;
> I don't understand why you need this?

It's part of the standard perf stuff. In perf_init_event() we do an idr 
lookup to identify the responsible PMU, and it that fails the PMUs get 
iterated over and need to return -ENOENT for events they don't own. That 
said, I have no idea what cases the idr lookup would fail.

>> +	/* config2 is unused */
>> +	if (event->attr.config2)
>> +		return -EINVAL;
>
> You must also check the reserved regions of config and config1.

There aren't any (I use all the bits in config and config1).

>> +	/* unsupported modes and filters */
>> +	if (event->attr.exclude_user   ||
>> +	    event->attr.exclude_kernel ||
>> +	    event->attr.exclude_hv     ||
>> +	    event->attr.exclude_idle   ||
>> +	    event->attr.exclude_host   ||
>> +	    event->attr.exclude_guest  ||
>> +	    is_sampling_event(event)) /* no sampling */
>
> I think you should also check sample_type.

Why? Many PMUs don't, I'm not seeing what the need is here.

>> +		return -EINVAL;
>
> Have you thought about inherit, pinned, exclusive?
>

exclusive and pinned don't make any sense because we don't have any 
actual PMU hw that is being consumed. The event counters are always 
counting.
inherit isn't relevant because we forbid all task tracking (via "+ 
.task_ctx_nr = perf_invalid_context," below).

I haven't forbidden either of these because I'm not sure there is a 
point in doing so (the exclude_* stuff is used for some permissions 
checking)

>> +
>> +	/* no branch sampling */
>> +	if (has_branch_stack(event))
>> +		return -EOPNOTSUPP;
>> +
>> +	length = event_get_length(event);
>> +	if (length < 1 || length > 8)
>> +		return -EINVAL;
>> +
>> +	/* last byte within the buffer? */
>> +	if ((event_get_offset(event) + length) > sizeof(union h_gpci_cvs))
>> +		return -EINVAL;
>> +
>> +	/* check if the request works... */
>> +	if (single_gpci_request(event_get_request(event),
>> +				event_get_starting_index(event),
>> +				event_get_secondary_index(event),
>> +				event_get_counter_info_version(event),
>> +				event_get_offset(event),
>> +				length,
>> +				&count))
>> +		return -EINVAL;
>> +
>> +	/*
>> +	 * Some of the events are per-cpu, some per-core, some per-chip, some
>> +	 * are global, and some access data from other virtual machines on the
>> +	 * same physical machine. We can't map the cpu value without a lot of
>> +	 * work. Instead, we pick an arbitrary cpu for all events on this pmu.
>> +	 */
>> +	event->cpu = 0;
>
> OK, but is having them all on cpu zero a good idea?

Probably not. We can remove this line without any real effect on the 
event output. Ideally, we'd have a way to tell perf that our PMU isn't 
associated with _any_ cpu (which would also eliminate some cross-cpu 
calls which trigger IPIs), and that is something I'm looking at, but 
perf widely assumes that events are tied to cpus (or some cpu mappable 
unit like a core or socket).

>
>> +	perf_swevent_init_hrtimer(event);
>> +	return 0;
>> +}
>> +
>> +struct pmu h_gpci_pmu = {
>> +	.task_ctx_nr = perf_invalid_context,
>> +
>> +	.name = "hv_gpci",
>> +	.attr_groups = attr_groups,
>> +	.event_init = h_gpci_event_init,
>> +	.add = h_gpci_event_add,
>> +	.del = h_gpci_event_del,
>> +	.start = h_gpci_event_start,
>> +	.stop = h_gpci_event_stop,
>> +	.read = h_gpci_event_read,
>
> Nice to have them align vertically.
Ack.

>
>> +	.event_idx = perf_swevent_event_idx,
>> +};
>> +
>> +static int hv_gpci_init(void)
>> +{
>> +	int r;
>> +
>> +	if (!firmware_has_feature(FW_FEATURE_LPAR)) {
>> +		pr_info("Not running under phyp, not supported\n");
>
> If only it was that simple :)
>
> You'll see FW_FEATURE_LPAR in a KVM guest too.
>
> There are at least two mechanisms for FW to indicate the presence of features,
> "ibm,hypertas-functions" and "ibm,architecture-vec-5".

I don't _think_ it is exposed in either of those, or if it is the docs 
don't say so :(.

> If HGPCI is not exposed in either of those then we'd want to do a probe hcall
> here to try and detect it at runtime.

Yep, having a probing hcall would be the way to go. The only issue there 
is handling migration between firmware which has or doesn't have support 
for this hcall. I suppose hooking a migration notifier of some kind 
would let us get past that bump.

>> +		return -ENODEV;
>> +	}
>> +
>> +	r = perf_pmu_register(&h_gpci_pmu, h_gpci_pmu.name, -1);
>> +	if (r)
>> +		return r;
>> +
>> +	return 0;
>> +}
>> +
>> +module_init(hv_gpci_init);
>
> This is not modular code so you're discouraged from using module_init(),
> arch_initcall() would probably be fine.
>

Got it.