Re: [PATCH v2] semihosting/arm-compat: remove heuristic softmmu SYS_HEAPINFO

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 10 Jun 2021 at 11:26, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> The previous numbers were a guess at best. While we could extract the
> information from a loaded ELF file via -kernel we could still get
> tripped up by self decompressing or relocating code. Besides sane
> library code like newlib will fall back to known symbols to determine
> of the location of the heap. We can still report the limits though as
> we are reasonably confident that busting out of RAM would be a bad
> thing for either stack or heap.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Tested-by: Andrew Strauss <astrauss11@gmail.com>
> Reviewed-by: Andrew Strauss <astrauss11@gmail.com>
> Message-Id: <20210601090715.22330-1-alex.bennee@linaro.org>
>
> ---
> v2
>   - report some known information (limits)
>   - reword the commit message
> ---
>  semihosting/arm-compat-semi.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/semihosting/arm-compat-semi.c b/semihosting/arm-compat-semi.c
> index 1c29146dcf..8873486e8c 100644
> --- a/semihosting/arm-compat-semi.c
> +++ b/semihosting/arm-compat-semi.c
> @@ -1202,10 +1202,14 @@ target_ulong do_common_semihosting(CPUState *cs)
>              retvals[3] = 0; /* Stack limit.  */
>  #else
>              limit = current_machine->ram_size;
> -            /* TODO: Make this use the limit of the loaded application.  */
> -            retvals[0] = rambase + limit / 2;
> -            retvals[1] = rambase + limit;
> -            retvals[2] = rambase + limit; /* Stack base */
> +            /*
> +             * Reporting 0 indicates we couldn't calculate the real
> +             * values which should force most software to fall back to
> +             * using information it has.
> +             */
> +            retvals[0] = 0; /* Heap Base */
> +            retvals[1] = rambase + limit; /* Heap Limit */
> +            retvals[2] = 0; /* Stack base */
>              retvals[3] = rambase; /* Stack limit.  */

The spec:
https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst#sys-heapinfo-0x16

doesn't document that 'return 0 for "I don't know"' is valid, so if we're
going to do it we ought to at least note that we're deviating from the
spec here.

-- PMM