SBI Debug Console Extension Proposal (Draft v1)

[Anup Patel]

On Thu, Jun 2, 2022 at 10:08 AM Xiang W <wxjstz at 126.com> wrote:
>
> 在 2022-06-01星期三的 21:47 +0530，Anup Patel写道：
> > 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`
> > 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.
> >
>
> Shared memory can be a single extension. The three interfaces are as follows
>
> /* The supervisor hands a piece of physical memory to sbi for shared memory */
> struct sbiret sbi_shared_memory_extrend(unsigned long addr, unsigned long size);
>
>
> /* The supervisor applies for a piece of physical memory from sbi */
> struct sbiret sbi_shared_memory_alloc(unsigned long size);
>
>
> /* The supervisor notifies sbi to release the requested memory */
> struct sbiret sbi_shared_memory_free(unsigned long addr);

Clearly, if we have a separate SBI extension to manage shared memory
then we will end-up with such memory management calls. Memory allocators
are generally hard to get it right and this also adds lot of bookkeeping and
state management in SBI implementations (e.g. OpenSBI, KVM RISC-V,
and various hypervisors).

Further, some of the SBI extensions (such as Steal Time Accounting) will
have separate shared memory for each HART whereas some (such as
Debug Console) will have global shared memory for all HARTs.

For clean and modular SBI implementations, I would recommend that
each SBI extension define its own shared memory setup API.

Regards,
Anup

>
>
> > 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.
> >
>
>