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

[Peter Maydell]

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.  */
>  #endif

I'm told that the Arm C compiler C library always assumes that
the "stack base" value is what it should set SP to, so reporting 0
for that will break binaries that were built with it.

As the TODO comment notes, the "heap base" is a bit of a guess,
but putting stackbase at top-of-RAM seems generally sensible.

What bug are we trying to fix here?

I think one possible implementation that might not be too
hard to make work would be:

 (1) find the guest physical address of the main machine
     RAM (machine->ram). You can do this with flatview_for_each_range()
     similar to what rom_ptr_for_as() does. (It might be mapped
     more than once, we could just pick the first one.)
 (2) find the largest contiguous extent of that RAM which
     is not covered by a ROM blob, by iterating through the
     ROM blob data. (This sounds like one of those slightly
     irritating but entirely tractable algorithms questions :-))
 (3) report the lowest address of that extent as the heap base
     and the stack limit, and the highest address as the heap
     limit and the stack base.

This would work for all guest architectures and remove the need
for that Arm-specific code...

-- PMM