Subject: Re: [PATCH v3 2/3] powerpc/pseries: Export hardware trace macro dump via debugfs

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Fri, 20 Sep 2024 21:45:55 +0530

Madhavan Srinivasan <[email-masked]> writes:

> This patch adds debugfs interface to export Hardware Trace Macro (HTM)
> function data in a LPAR. New hypervisor call "H_HTM" has been
> defined to setup, configure, control and dump the HTM data.
> This patch supports only dumping of HTM data in a LPAR.
> New debugfs folder called "htmdump" has been added under
> /sys/kernel/debug/arch path which contains files need to
> pass required parameters for the H_HTM dump function. New Kconfig
> option called "CONFIG_HTMDUMP" has been in platform/pseries for the same.
>
> With this module loaded, list of files in debugfs path
>
> /sys/kernel/debug/powerpc/htmdump
> coreindexonchip  htmtype  nodalchipindex  nodeindex  trace
>
> Signed-off-by: Madhavan Srinivasan <[email-masked]>
> ---
> Changelog v2:
> - Made driver as modules based on review comments
> Changelog v1:
> - Changed from tristate to bool with dependency flags
> - Trimmed the include headers
>
>  arch/powerpc/platforms/pseries/Kconfig   |   9 ++
>  arch/powerpc/platforms/pseries/Makefile  |   1 +
>  arch/powerpc/platforms/pseries/htmdump.c | 130 +++++++++++++++++++++++
>  3 files changed, 140 insertions(+)
>  create mode 100644 arch/powerpc/platforms/pseries/htmdump.c
>
> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> index afc0f6a61337..a66be66d690e 100644
> --- a/arch/powerpc/platforms/pseries/Kconfig
> +++ b/arch/powerpc/platforms/pseries/Kconfig
> @@ -128,6 +128,15 @@ config CMM
>  	  will be reused for other LPARs. The interface allows firmware to
>  	  balance memory across many LPARs.
>  
> +config HTMDUMP
> +	tristate "PHYP HTM data dumper"
> +	depends on PPC_PSERIES && DEBUG_FS
> +	default m
> +	help
> +	  Select this option, if you want to enable the kernel debugfs
> +	  interface to dump the Hardware Trace Macro (HTM) function data
> +	  in the LPAR.
> +
>  config HV_PERF_CTRS
>  	bool "Hypervisor supplied PMU events (24x7 & GPCI)"
>  	default y
> diff --git a/arch/powerpc/platforms/pseries/Makefile b/arch/powerpc/platforms/pseries/Makefile
> index 7bf506f6b8c8..3f3e3492e436 100644
> --- a/arch/powerpc/platforms/pseries/Makefile
> +++ b/arch/powerpc/platforms/pseries/Makefile
> @@ -19,6 +19,7 @@ obj-$(CONFIG_HVC_CONSOLE)	+= hvconsole.o
>  obj-$(CONFIG_HVCS)		+= hvcserver.o
>  obj-$(CONFIG_HCALL_STATS)	+= hvCall_inst.o
>  obj-$(CONFIG_CMM)		+= cmm.o
> +obj-$(CONFIG_HTMDUMP)		+= htmdump.o
>  obj-$(CONFIG_IO_EVENT_IRQ)	+= io_event_irq.o
>  obj-$(CONFIG_LPARCFG)		+= lparcfg.o
>  obj-$(CONFIG_IBMVIO)		+= vio.o
> diff --git a/arch/powerpc/platforms/pseries/htmdump.c b/arch/powerpc/platforms/pseries/htmdump.c
> new file mode 100644
> index 000000000000..54c28525c4a7
> --- /dev/null
> +++ b/arch/powerpc/platforms/pseries/htmdump.c
> @@ -0,0 +1,130 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (C) IBM Corporation, 2024
> + */
> +
> +#define pr_fmt(fmt) "htmdump: " fmt
> +
> +#include <linux/debugfs.h>
> +
> +#include <asm/machdep.h>
> +#include <asm/plpar_wrappers.h>
> +
> +/* This enables us to keep track of the memory removed from each node. */
> +struct htmdump_entry {
> +	void *buf;
> +	struct dentry *dir;
> +	char name[16];
> +};
>

How does dir and name gets used?
It isn't that obvious, so maybe a comment will be gr8.

> +static u32 nodeindex;
> +static u32 nodalchipindex;
> +static u32 coreindexonchip;
> +static u32 htmtype;
> +static struct dentry *htmdump_debugfs_dir;
> +static struct htmdump_entry *ent;
> +
> +#define BUFFER_SIZE PAGE_SIZE
> +
> +static ssize_t htmdump_read(struct file *filp, char __user *ubuf,
> +			     size_t count, loff_t *ppos)
> +{
> +	struct htmdump_entry *ent = filp->private_data;
> +	unsigned long page, read_size, available;
> +	loff_t offset;
> +	long rc;
> +
> +	page = ALIGN_DOWN(*ppos, BUFFER_SIZE);
> +	offset = (*ppos) % BUFFER_SIZE;
> +
> +	rc = htm_get_dump_hardware(nodeindex, nodalchipindex, coreindexonchip,
> +				   htmtype, virt_to_phys(ent->buf), BUFFER_SIZE, page);
> +
> +	switch (rc) {
> +	case H_SUCCESS:
> +	case H_PARTIAL:
> +		break;
> +	case H_NOT_AVAILABLE:
> +		return 0;

Minor nits for error returns here...

Is returning 0 correct here? Maybe it is (since 0 means no data read),
but wanted to confirm if we should return -ENODATA, or -ENODEV 
(not sure what does H_NOT_AVAILABLE here means)

#define	ENODATA		61	/* No data available */

> +	case H_BUSY:
> +	case H_LONG_BUSY_ORDER_1_MSEC:
> +	case H_LONG_BUSY_ORDER_10_MSEC:
> +	case H_LONG_BUSY_ORDER_100_MSEC:
> +	case H_LONG_BUSY_ORDER_1_SEC:
> +	case H_LONG_BUSY_ORDER_10_SEC:
> +	case H_LONG_BUSY_ORDER_100_SEC:

Similarly for above maybe -EBUSY perhaps, instead of -EINVAL

#define	EBUSY		16	/* Device or resource busy */

> +	case H_PARAMETER:
> +	case H_P2:
> +	case H_P3:
> +	case H_P4:
> +	case H_P5:
> +	case H_P6:
> +	case H_STATE:
> +	case H_AUTHORITY:
> +		return -EINVAL;
> +	}
> +
> +	available = BUFFER_SIZE - offset;
> +	read_size = min(count, available);
> +	*ppos += read_size;
> +	return simple_read_from_buffer(ubuf, count, &offset, ent->buf, available);
> +}
> +
> +static const struct file_operations htmdump_fops = {
> +	.llseek = default_llseek,
> +	.read	= htmdump_read,
> +	.open	= simple_open,
> +};
> +
> +static int htmdump_init_debugfs(void)
> +{
> +	ent = kcalloc(1, sizeof(struct htmdump_entry), GFP_KERNEL);
> +	if (!ent) {
> +		pr_err("Failed to allocate ent\n");
> +		return -EINVAL;

return value can be -ENOMEM;

> +	}
> +
> +	ent->buf = kmalloc(BUFFER_SIZE, GFP_KERNEL);
> +	if (!ent->buf) {
> +		pr_err("Failed to allocate htmdump buf\n");
> +		return -ENOMEM;

kfree(ent)?

> +	}
> +
> +	pr_debug("%s: ent:%lx buf:%lx\n",
> +			__func__, (long unsigned int)ent, (long unsigned int)ent->buf);
> +

maybe %p perhaps? 

<Documentation/core-api/printk-formats.rst>
Plain Pointers
--------------

::

	%p	abcdef12 or 00000000abcdef12

Pointers printed without a specifier extension (i.e unadorned %p) are
hashed to prevent leaking information about the kernel memory layout. This
has the added benefit of providing a unique identifier.
<...>
...the aim of printing the address is to provide
more information for debugging, use %p and boot the kernel with the
``no_hash_pointers`` parameter during debugging, which will print all %p
addresses unmodified. If you *really* always want the unmodified address, see
%px below.

> +	htmdump_debugfs_dir = debugfs_create_dir("htmdump",
> +						  arch_debugfs_dir);
> +
> +	debugfs_create_u32("nodeindex", 0600,
> +			htmdump_debugfs_dir, &nodeindex);
> +	debugfs_create_u32("nodalchipindex", 0600,
> +			htmdump_debugfs_dir, &nodalchipindex);
> +	debugfs_create_u32("coreindexonchip", 0600,
> +			htmdump_debugfs_dir, &coreindexonchip);
> +	debugfs_create_u32("htmtype", 0600,
> +			htmdump_debugfs_dir, &htmtype);
> +	debugfs_create_file("trace", 0400, htmdump_debugfs_dir, ent, &htmdump_fops);
> +
> +	return 0;
> +}
> +
> +static int __init htmdump_init(void)
> +{
> +	if (htmdump_init_debugfs())
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +static void __exit htmdump_exit(void)
> +{
> +	debugfs_remove_recursive(htmdump_debugfs_dir);
> +	kfree(ent->buf);
> +	kfree(ent);
> +}
> +
> +module_init(htmdump_init);
> +module_exit(htmdump_exit);
> +MODULE_DESCRIPTION("PHYP Hardware Trace Macro (HTM) data dumper");
> +MODULE_LICENSE("GPL");
> -- 
> 2.45.2