[Pdbg] [RFC] main: Implement 'dmesg' command

[Balbir Singh]

On Fri, 25 May 2018 12:53:04 +0930
Joel Stanley <joel at jms.id.au> wrote:

> This command fetches the linux kernel log from host RAM and displays it
> on stdout.
> 
> Almost any powernv kernel will support this. However it requires a
> skiboot change in order to find the location of the kernel log buffer.
> On systems without that change this command will return a message saying
> it is unsupported.
> 
> Signed-off-by: Joel Stanley <joel at jms.id.au>
> ---
> The guts of the log parsing comes from benh's kmsg_dump.c.
> 
> The skiboot change has not been merged yet:
> 
>  http://patchwork.ozlabs.org/patch/919508/
> 
> More importantly, this doesn't currently work, as we cannot read the
> memory region that log_buf points to. Jump to the end of this essay to
> see the error.
> 
> __log_buf[] is a statically allocated buffer that the kernel uses for
> it's messages. However, we refer to it using the pointer log_buf, as the
> kernel may chose to reallocate the buffer.
> 
> kernel/printk/printk.c:
> 
>         static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
>         static char *log_buf = __log_buf;
>         static u32 log_buf_len = __LOG_BUF_LEN;
> 
>         /* Return log buffer address */
>         char *log_buf_addr_get(void)
>         {
>                 return log_buf;
>         }
> 
>         ...
> 
> 	if (early) {
>                 new_log_buf =
>                         memblock_virt_alloc(new_log_buf_len, LOG_ALIGN);
>         } else {
>                 new_log_buf = memblock_virt_alloc_nopanic(new_log_buf_len,
>                                                           LOG_ALIGN);
>         }
> 
> The value gets retrieved via log_buf_addr_get in the powernv platform
> code, and sent to skiboot via the opal_register_dump_region OPAL call.
> 
> arch/powerpc/platforms/powernv/opal.c
> 
>         addr = log_buf_addr_get();
>         if (addr == NULL)
>                 return;
>         ...
> 
>         rc = opal_register_dump_region(OPAL_DUMP_REGION_LOG_BUF,
>                                        __pa(addr), size);
> 
> Skiboot sticks this in the debug descriptor:
> 
> core/dump-region.c
> 
>         static int64_t bmc_opal_register_dump_region(uint32_t id,
>                                                      uint64_t addr, uint64_t size)
>         {
>                 ...
>                 prlog(PR_INFO, "Registered log buf at 0x%016llx\n", addr);
>                 debug_descriptor.log_buf_phys = addr;
> 
> Looking at the host's kallsyms, we can see where __log_buf starts. If we
> hardcode that as the address to read from in pdbg, we can get data out:
>

Let's not hardcode, please. Have you tried this with CONFIG_RELOCATABLE
and CONFIG_RELOCATABLE_TEST set?

Balbir Singh.