[coreboot] Re: [SPECIFICATION RFC] The firmware and bootloader log specification

[Wim Vervoorn]

Hello Julius,

I agree with you. Using an existing standard is better than inventing a new one in this case. I think using the coreboot logging is a good idea as there is indeed a lot of support already available and it is lightweight and simple.

Best Regards,
Wim Vervoorn

Eltan B.V.
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-----Original Message-----
From: Grub-devel [mailto:grub-devel-bounces+wvervoorn=[e-mail]] On Behalf Of Julius Werner
Sent: Thursday, December 3, 2020 2:18 AM
To: Daniel Kiper 
Cc: Coreboot ; The development of GRUB 2 ; LKML ; [e-mail]; [e-mail]; U-Boot Mailing List ; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; Vladimir 'phcoder' Serbinenko ; [e-mail]; [e-mail]; Paul Menzel ; [e-mail]; [e-mail]; [e-mail]; Heinrich Schuchardt 
Subject: Re: [SPECIFICATION RFC] The firmware and bootloader log specification

Standardizing in-memory logging sounds like an interesting idea, especially with regards to components that can run on top of different firmware stacks (things like GRUB or TF-A). But I would be a bit wary of creating a "new standard to rule them all" and then expecting all projects to switch what they have over to that. I think we all know https://xkcd.com/927/.

Have you looked around and evaluated existing solutions that already have some proliferation first? I think it would be much easier to convince people to standardize on something that already has existing users and drivers available in multiple projects.

In coreboot we're using a very simple character ring buffer that only has two 4-byte header fields: total size and cursor (i.e. current position where you would write the next character). The top 4 bits of the cursor field are reserved for flags, one of which is the "overflow" flag that tells you whether the ring-buffer already overflowed or not (so readers know whether to print the whole ring buffer, or only from the start to the current cursor). We try to dimension the buffers so they don't overflow on a single boot, but this approach allows us to log multiple boots on platforms that can persist memory across reboots, which sometimes comes in handy.

The disadvantages of that approach compared to your proposal are lack of some features, like the facilities field (although one can still just print a tag like "<0>" or "<4>" behind each newline) or timestamps (coreboot instead has separate timestamp logging). But I think a really big advantage is size: in early firmware environments before DDR training, space is often very precious and we struggle to find more than a couple of kilobytes for the log buffer. If I look at the structure you proposed, that's already 24 bytes of overhead per individual message. If we were hooking that up to our normal printk() facility in coreboot (such that each invocation creates a new message header), that would probably waste about a third of the whole log buffer on overhead. I think a complicated, syslog-style logging structure that stores individual message blocks instead of a continuous character string isn't really suitable for firmware logging.

FWIW the coreboot console has existing support in Linux (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/driv...),
SeaBIOS (https://github.com/coreboot/seabios/blob/master/src/fw/coreboot.c#L219),
TF-A (https://github.com/ARM-software/arm-trusted-firmware/blob/master/drivers/cor...),
GRUB (https://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/term/i386/coreboot...),
U-Boot (https://github.com/u-boot/u-boot/blob/master/drivers/misc/cbmem_console.c)
and probably a couple of others I'm not aware of. And the code to add support (especially when only appending) is so simple that it just takes a couple of lines to implement (binary code size to implement the format is also always a concern for firmware environments).

On Wed, Nov 18, 2020 at 7:04 AM Heinrich Schuchardt  wrote:
> On 14.11.20 00:52, Daniel Kiper wrote:
> > Hey,
> >
> > This is next attempt to create firmware and bootloader log specification.
> > Due to high interest among industry it is an extension to the 
> > initial bootloader log only specification. It takes into the account 
> > most of the comments which I got up until now.
> >
> > The goal is to pass all logs produced by various boot components to 
> > the running OS. The OS kernel should expose these logs to the user 
> > space and/or process them internally if needed. The content of these 
> > logs should be human readable. However, they should also contain the 
> > information which allows admins to do e.g. boot time analysis.
> >
> > The log specification should be as much as possible platform 
> > agnostic and self contained. The final version of this spec should 
> > be merged into existing specifications, e.g. UEFI, ACPI, Multiboot2, 
> > or be a standalone spec, e.g. as a part of OASIS Standards. The 
> > former seems better but is not perfect too...
> >
> > Here is the description (pseudocode) of the structures which will be 
> > used to store the log data.
>
> Hello Daniel,
>
> thanks for your suggestion which makes good sense to me.
>
> Why can't we simply use the message format defined in "The Syslog 
> Protocol", https://tools.ietf.org/html/rfc5424?
> > struct bf_log
> >   {
> >     uint32_t   version;
> >     char       producer[64];
> >     uint64_t   flags;
> >     uint64_t   next_bf_log_addr;
> >     uint32_t   next_msg_off;
> >     bf_log_msg msgs[];
>
> As bf_log_msg is does not have defined length msgs[] cannot be an array.
> > }
> >
> > struct bf_log_msg
> >   {
> >     uint32_t size;
> >     uint64_t ts_nsec;
> >     uint32_t level;
> >     uint32_t facility;
> >     uint32_t msg_off;
> >     char     strings[];
> >   }
> >
> > The members of struct bf_log:
> > version: the firmware and bootloader log format version number, 1 for now,
> > producer: the producer/firmware/bootloader/... type; the length
> > allows ASCII UUID storage if somebody needs that functionality,
> > flags: it can be used to store information about log state, e.g.
> > it was truncated or not (does it make sense to have an information
> > about the number of lost messages?),
> > next_bf_log_addr: address of next bf_log struct; none if zero (I think
> > newer spec versions should not change anything in first 5 bf_log members;
> > this way older log parsers will be able to traverse/copy all logs regardless
> > of version used in one log or another),
> > next_msg_off: the offset, in bytes, from the beginning of the bf_log struct,
> > of the next byte after the last log message in the msgs[]; i.e. the offset
> > of the next available log message slot; it is equal to the total size of
> > the log buffer including the bf_log struct,
>
> Why would you need an offset to first unused byte?
>
> We possibly have multiple producers of messages:
> TF-A
> U-Boot
> iPXE
> GRUB
>
> What we need is the offset to the next struct bf_log.
> > msgs: the array of log messages,
> > should we add CRC or hash or signatures here?
> >
> > The members of struct bf_log_msg:
> > size: total size of bf_log_msg struct,
> > ts_nsec: timestamp expressed in nanoseconds starting from 0,
>
> Would each message producer start from 0?
>
> Shouldn't we use the time from the hardware RTC if it is available via 
> boot service GetTime()?
> > level: similar to syslog meaning; can be used to differentiate normal messages
> > from debug messages; the exact interpretation depends on the current producer
> > type specified in the bf_log.producer,
> > facility: similar to syslog meaning; can be used to differentiate the sources of
> > the messages, e.g. message produced by networking module; the exact interpretation
> > depends on the current producer type specified in the bf_log.producer,
> > msg_off: the log message offset in strings[],
>
> What is this field good for? Why don't you start the the string at 
> strings[0]?
> What would be useful would be the offset to the next bf_log_msg.
> > strings[0]: the beginning of log message type, similar to the facility member but
> > NUL terminated string instead of integer; this will be used by, e.g., the GRUB2
> > for messages printed using grub_dprintf(),
> > strings[msg_off]: the beginning of log message, NUL terminated string.
>
> Why strings in plural? Do you want to put multiple strings into 
> 'strings'? What identifies the last string?
> > Note: The producers are free to use/ignore any given set of level, facility and/or
> >       log type members. Though the usage of these members has to be clearly defined.
> >       Ignored integer members should be set to 0. Ignored log message type should
> >       contain an empty NUL terminated string. The log message is mandatory but can
> >       be an empty NUL terminated string.
> >
> > There is still not fully solved problem how the logs should be presented to the OS.
> > On the UEFI platforms we can use config tables to do that. Then 
> > probably bf_log.next_bf_log_addr should not be used.
>
> Why? How would you otherwise find the entries of the next produser in 
> the configuration table? What I am missing is a GUID for the 
> configuration table.
> > On the ACPI and Device Tree platforms we can use these mechanisms to 
> > present the logs to the OSes. The situation gets more
>
> I do not understand this.
>
> UEFI implementations use either of ACPI and device-trees and support 
> configuration tables. Why do you want to use some other binding?
>
> Best regards
>
> Heinrich
> > difficult if neither of these mechanisms are present. However, maybe 
> > we should not bother too much about that because probably these 
> > platforms getting less and less common.
> >
> > Anyway, I am aware that this is not specification per se. The goal 
> > of this email is to continue the discussion about the idea of the 
> > firmware and booloader log and to find out where the final 
> > specification should land. Of course taking into the account assumptions made above.
> >
> > You can find previous discussions about related topics at [1], [2] and [3].
> >
> > Additionally, I am going to present this during GRUB mini-summit 
> > session on Tuesday, 17th of November at 15:45 UTC. So, if you want 
> > to discuss the log design please join us. You can find more details here [4].
> >
> > Daniel
> >
> > [1] 
> > https://lists.gnu.org/archive/html/grub-devel/2019-10/msg00107.html
> > [2] 
> > https://lists.gnu.org/archive/html/grub-devel/2019-11/msg00079.html
> > [3] 
> > https://lists.gnu.org/archive/html/grub-devel/2020-05/msg00223.html
> > [4] https://twitter.com/3mdeb_com/status/1327278804100931587

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