Re: [PATCH 0/8] tracing: Persistent traces across a reboot or crash

[Kees Cook]

On Sat, Mar 09, 2024 at 01:51:16PM -0500, Steven Rostedt wrote:
> On Sat, 9 Mar 2024 10:27:47 -0800
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
> > On Tue, Mar 05, 2024 at 08:59:10PM -0500, Steven Rostedt wrote:
> > > This is a way to map a ring buffer instance across reboots.  
> > 
> > As mentioned on Fedi, check out the persistent storage subsystem
> > (pstore)[1]. It already does what you're starting to construct for RAM
> > backends (but also supports reed-solomon ECC), and supports several
> > other backends including EFI storage (which is default enabled on at
> > least Fedora[2]), block devices, etc. It has an existing mechanism for
> > handling reservations (including via device tree), and supports multiple
> > "frontends" including the Oops handler, console output, and even ftrace
> > which does per-cpu recording and event reconstruction (Joel wrote this
> > frontend).
> 
> Mathieu was telling me about the pmem infrastructure.

I use nvdimm to back my RAM backend testing with qemu so I can examine
the storage "externally":

RAM_SIZE=16384
NVDIMM_SIZE=200
MAX_SIZE=$(( RAM_SIZE + NVDIMM_SIZE ))
..
qemu-system-x86_64 \
	...
        -machine pc,nvdimm=on \
        -m ${RAM_SIZE}M,slots=2,maxmem=${MAX_SIZE}M \
        -object memory-backend-file,id=mem1,share=on,mem-path=$IMAGES/x86/nvdimm.img,size=${NVDIMM_SIZE}M,align=128M
\
        -device nvdimm,id=nvdimm1,memdev=mem1,label-size=1M \
	...
        -append 'console=uart,io,0x3f8,115200n8 loglevel=8 root=/dev/vda1 ro ramoops.mem_size=1048576 ramoops.ecc=1 ramoops.mem_address=0x440000000 ramoops.console_size=16384 ramoops.ftrace_size=16384 ramoops.pmsg_size=16384 ramoops.record_size=32768 panic=-1 init=/root/resume.sh '"$@"


The part I'd like to get wired up sanely is having pstore find the
nvdimm area automatically, but it never quite happened:
https://lore.kernel.org/lkml/CAGXu5jLtmb3qinZnX3rScUJLUFdf+pRDVPjy=CS4KUtW9tLHtw@xxxxxxxxxxxxxx/

> Thanks for the info. We use pstore on ChromeOS, but it is currently
> restricted to 1MB which is too small for the tracing buffers. From what
> I understand, it's also in a specific location where there's only 1MB
> available for contiguous memory.

That's the area that is specifically hardware backed with persistent
RAM.

> I'm looking at finding a way to get consistent memory outside that
> range. That's what I'll be doing next week ;-)
> 
> But this code was just to see if I could get a single contiguous range
> of memory mapped to ftrace, and this patch set does exactly that.

Well, please take a look at pstore. It should be able to do everything
you mention already; it just needs a way to define multiple regions if
you want to use an area outside of the persistent ram area defined by
Chrome OS's platform driver.

-Kees

-- 
Kees Cook