SBI Debug Console Extension Proposal (Draft v1)

[Anup Patel]

On Thu, Jun 2, 2022 at 2:58 PM Heiko Stübner <heiko at sntech.de> wrote:
>
> Am Donnerstag, 2. Juni 2022, 10:50:56 CEST schrieb Heiko Stübner:
> > Am Donnerstag, 2. Juni 2022, 10:47:58 CEST schrieb Anup Patel:
> > > On Thu, Jun 2, 2022 at 12:02 AM Heiko Stübner <heiko at sntech.de> wrote:
> > > >
> > > > Hi Anup,
> > > >
> > > > Am Mittwoch, 1. Juni 2022, 18:17:32 CEST schrieb Anup Patel:
> > > > > Hi All,
> > > > >
> > > > > Below is the draft proposal for SBI Debug Console Extension.
> > > > >
> > > > > Please review it and provide feedback.
> > > > >
> > > > > Thanks,
> > > > > Anup
> > > > >
> > > > > Debug Console Extension (EID #0x4442434E "DBCN")
> > > > > ================================================
> > > > >
> > > > > The debug console extension defines a generic mechanism for boot-time
> > > > > early prints from supervisor-mode software which allows users to catch
> > > > > boot-time issues in supervisor-mode software.
> > > > >
> > > > > This extension replaces legacy console putchar (EID #0x01) extension
> > > > > and it is better in following ways:
> > > > > 1) It follows the new calling convention defined for SBI v1.0
> > > > >    (or higher).
> > > > > 2) It is based on a shared memory area between SBI implementation
> > > > >    and supervisor-mode software so multiple characters can be
> > > > >    printed using a single SBI call.
> > > > >
> > > > > The supervisor-mode software must set the shared memory area before
> > > > > printing characters on the debug console. Also, all HARTs share the
> > > > > same shared memory area so only one HART needs to set it at boot-time.
> > > > >
> > > > > Function: Set Console Area (FID #0)
> > > > > -----------------------------------
> > > > >
> > > > > struct sbiret sbi_debug_console_set_area(unsigned long addr_div_by_4,
> > > > >                                          unsigned long size)
> > > > >
> > > > > Set the shared memory area specified by `addr_div_by_2` and `size`
> > > >
> > > > typo in the "div_by_2" (not 4 like below and in the function itself) ?
> > > >
> > > >
> > > > > parameters. The `addr_div_by_4` parameter is base address of the
> > > > > shared memory area right shifted by 2 whereas `size` parameter is
> > > > > the size of shared memory area in bytes.
> > > > >
> > > > > The shared memory area should be normal cacheable memory for the
> > > > > supervisor-mode software. Also, the shared memory area is global
> > > > > across all HARTs so SBI implementation must ensure atomicity in
> > > > > setting the shared memory area.
> > > > >
> > > > > Errors:
> > > > > SBI_SUCCESS                - Shared memory area set successfully.
> > > > > SBI_ERR_INVALID_ADDRESS - The shared memory area pointed by
> > > > >                           `addr_div_by_2` and `size` parameters
> > > > >                           is not normal cacheable memory or not
> > > > >                           accessible to supervisor-mode software.
> > > > >
> > > > > Function: Console Puts (FID #1)
> > > > > -------------------------------
> > > > >
> > > > > struct sbiret sbi_debug_console_puts(unsigned long area_offset,
> > > > >                                      unsigned long num_chars)
> > > > >
> > > > > Print the string specified by `area_offset` and `num_chars` on
> > > > > the debug console. The `area_offset` parameter is the start of
> > > > > string in the shard memory area whereas `num_chars` parameter
> > > > > is the number of characters (or bytes) in the string.
> > > > >
> > > > > This is a blocking SBI call and will only return after printing
> > > > > all characters of the string.
> > > > >
> > > > > Errors:
> > > > > SBI_SUCCESS                - Characters printed successfully.
> > > > > SBI_ERR_INVALID_ADDRESS    - The start of the string (i.e.
> > > > >                           `area_offset`) or end of the string
> > > > >                           (i.e. `area_offset + num_chars`) is
> > > > >                           outside shared memory area.
> > > >
> > > > This will vastly reduce the number of needed ecalls when outputting
> > > > characters, so this will probably improve performance quite a bit :-)
> > > >
> > > >
> > > > I guess I still would like to have an _additional_ single-character
> > > > putc call. As mentioned in the other thread [0], especially on consumer
> > > > hardware [where there is no elaborate debug infrastructure] this can
> > > > be a very handy debugging tool even in the earliest stages of a
> > > > booting kernel (both before relocation and even inside the startup
> > > > assembly).
> > > >
> > > > I.e. just doing a
> > > >         li a7, 1
> > > >         li a6, 0
> > > >         li a0, 36
> > > >         ecall
> > > >
> > > > in any kernel assembly will just output a "$" character right now, without
> > > > needing any preparation at all - same with using the current
> > > > sbi_console_putchar() directly in c-code.
> > > >
> > > > This _can_ be very helpful in some cases, so I guess it would be nice
> > > > to keep such a functionality around also in the new spec.
> > >
> > > You can easily create multiple pre-populated strings using ".asciiz" in
> > > assembly sources. Just set the base address of pre-populated strings
> > > once on boot hart and print from anywhere using usual 4-5 instruction
> > > (similar to what posted above).
> >
> > ok, sounds like a plan as well :-)
>
> though, how does that relate to the time before MMU setup?
>
> I.e. in response to Heinrich's mail you talk about svpbmt, so I guess you
> expect virtual memory there, so what is the expected value-type before
> the mmu is setup in S-mode ?

The memory type should be 0 (i.e. PMA) for the shared memory between
SBI implementation and supervisor software. Before MMU setup, the
memory type is by default 0 (i.e. PMA) so we don't have to mandate
any memory type for MMU disabled case.

We only have issue on systems with Svpmbt where supervisor software
can potentially map the shared memory as non-cacheable or IO (memory
type != 0) using PTE memory type bits.

In addition to above, a SBI implementation must ensure that the shared
memory address provided by supervisor software is a regular memory
(not MMIO device). This can be easily achieved in OpenSBI, KVM RISC-V,
and various hypervisors.

Regards,
Anup

>
>
> > > > [0] http://lists.infradead.org/pipermail/opensbi/2022-June/002796.html
> > > >
> > > >
> > > >
> > >
> > > Regards,
> > > Anup
> > >
> >
> >
>
>
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