[PATCH] Introduce callchains for guests

[PATCH] Introduce callchains for guests

[Tianyi Liu]

This patch serves as the foundation for enabling callchains for guests,
(used by `perf kvm`). This functionality is useful for me, and I
noticed it holds the top spot on the perf wiki TODO list [1], so I'd like
to implement it. This patch introduces a `perf_callchain_guest` interface,
which different architectures can implement according to their own
specifications.

This is part of a series of patches. Since these patches are spread across
various modules like perf, kvm, arch/*, I plan to first submit some
foundational patches and gradually submit the complete implementation.
The full implementation can be found at [2], and it has been tested on
an x86_64 machine.

Since perf tools currently do not support callchain types with
PERF_CONTEXT_GUEST*, I temporarily used PERF_CONTEXT_KERNEL for ease of
testing and validation. This will be rectified once the perf tools patches
are applied.

Furthermore, do you think it's necessary to introduce a new guest filter
in the parameters of `get_perf_callchain`? Alternatively, should we add
two separate filters, `guest_user` and `guest_kernel`, to make these
filters entirely independent?

This is my first time submitting a patch for a new feature in the kernel,
and I would greatly appreciate any suggestions.

[1] https://perf.wiki.kernel.org/index.php/Todo
[2] https://github.com/i-Pear/linux/pull/2/files

Signed-off-by: Tianyi Liu <i.pear@outlook.com>
---
 kernel/events/callchain.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1273be84392c..eacb6ccf7034 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -45,6 +45,10 @@ __weak void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
 {
 }
 
+__weak void perf_callchain_guest(struct perf_callchain_entry_ctx *entry)
+{
+}
+
 static void release_callchain_buffers_rcu(struct rcu_head *head)
 {
 	struct callchain_cpus_entries *entries;
@@ -183,6 +187,7 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 	struct perf_callchain_entry *entry;
 	struct perf_callchain_entry_ctx ctx;
 	int rctx;
+	unsigned int guest_state;
 
 	entry = get_callchain_entry(&rctx);
 	if (!entry)
@@ -194,6 +199,26 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 	ctx.contexts       = 0;
 	ctx.contexts_maxed = false;
 
+	guest_state = perf_guest_state();
+	if (guest_state) {
+		if (add_mark) {
+			if (guest_state & PERF_GUEST_USER)
+				/*
+				 * TODO: Change this to PERF_CONTEXT_GUEST_USER,
+				 * which is currently not recognized by perf utils.
+				 */
+				perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
+			else
+				/*
+				 * TODO: Change this to PERF_CONTEXT_GUEST_KERNEL,
+				 * which is currently not recognized by perf utils.
+				 */
+				perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
+		}
+		perf_callchain_guest(&ctx);
+		goto exit_put;
+	}
+
 	if (kernel && !user_mode(regs)) {
 		if (add_mark)
 			perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
-- 
2.34.1

Re: [PATCH] Introduce callchains for guests

[kernel test robot]

Hi Tianyi,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tip/perf/core]
[also build test WARNING on acme/perf/core linus/master v6.5 next-20230831]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Tianyi-Liu/Introduce-callchains-for-guests/20230831-161333
base:   tip/perf/core
patch link:    https://lore.kernel.org/r/SY4P282MB10848199D3F93AA47A845B8B9DE5A%40SY4P282MB1084.AUSP282.PROD.OUTLOOK.COM
patch subject: [PATCH] Introduce callchains for guests
config: alpha-allyesconfig (https://download.01.org/0day-ci/archive/20230831/202308311743.uvv4Ras8-lkp@intel.com/config)
compiler: alpha-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230831/202308311743.uvv4Ras8-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202308311743.uvv4Ras8-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> kernel/events/callchain.c:48:13: warning: no previous prototype for 'perf_callchain_guest' [-Wmissing-prototypes]
      48 | __weak void perf_callchain_guest(struct perf_callchain_entry_ctx *entry)
         |             ^~~~~~~~~~~~~~~~~~~~


vim +/perf_callchain_guest +48 kernel/events/callchain.c

    47	
  > 48	__weak void perf_callchain_guest(struct perf_callchain_entry_ctx *entry)
    49	{
    50	}
    51	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki