[Crash-utility] new printk ringbuffer interface

[John Ogness]

Hello Dave,

You may or may not be aware that we are working on replacing [0] the
Linux printk ringbuffer. Rather than a single buffer containing a single
struct type, the new ringbuffer makes use of several different structs.

I am writing to ask your advice about how this should be exported for
the crash utility. Should all struct sizes and field offsets be
exported? It would look something like this:

        VMCOREINFO_SYMBOL(prb);

        VMCOREINFO_STRUCT_SIZE(printk_ringbuffer);
        VMCOREINFO_OFFSET(printk_ringbuffer, desc_ring);
        VMCOREINFO_OFFSET(printk_ringbuffer, text_data_ring);
        VMCOREINFO_OFFSET(printk_ringbuffer, dict_data_ring);
        VMCOREINFO_OFFSET(printk_ringbuffer, fail);

        VMCOREINFO_STRUCT_SIZE(prb_desc_ring);
        VMCOREINFO_OFFSET(prb_desc_ring, count_bits);
        VMCOREINFO_OFFSET(prb_desc_ring, descs);
        VMCOREINFO_OFFSET(prb_desc_ring, head_id);
        VMCOREINFO_OFFSET(prb_desc_ring, tail_id);

        VMCOREINFO_STRUCT_SIZE(prb_desc);
        VMCOREINFO_OFFSET(prb_desc, info);
        VMCOREINFO_OFFSET(prb_desc, state_var);
        VMCOREINFO_OFFSET(prb_desc, text_blk_lpos);
        VMCOREINFO_OFFSET(prb_desc, dict_blk_lpos);

        VMCOREINFO_STRUCT_SIZE(prb_data_blk_lpos);
        VMCOREINFO_OFFSET(prb_data_blk_lpos, begin);
        VMCOREINFO_OFFSET(prb_data_blk_lpos, next);

        VMCOREINFO_STRUCT_SIZE(printk_info);
        VMCOREINFO_OFFSET(printk_info, seq);
        VMCOREINFO_OFFSET(printk_info, ts_nsec);
        VMCOREINFO_OFFSET(printk_info, text_len);
        VMCOREINFO_OFFSET(printk_info, dict_len);
        VMCOREINFO_OFFSET(printk_info, caller_id);

        VMCOREINFO_STRUCT_SIZE(prb_data_ring);
        VMCOREINFO_OFFSET(prb_data_ring, size_bits);
        VMCOREINFO_OFFSET(prb_data_ring, data);
        VMCOREINFO_OFFSET(prb_data_ring, head_id);
        VMCOREINFO_OFFSET(prb_data_ring, tail_id);

Or would it be enough to just recognize the new "prb" symbol and have
all the structures defined in the crash utility? If the latter is
preferred, should some sort of version number be exported? Or is the
kernel version number enough?

I appreciate your feedback.

John Ogness

[0] lkml.kernel.org/r/20200128161948.8524-1-john.ogness at linutronix.de