Re: [PATCH v4 3/3] hw/riscv: change riscv_compute_fdt_addr() semantics

[Bin Meng <bmeng.cn@gmail.com>]

On Tue, Jan 31, 2023 at 1:16 AM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
>
>
>
> On 1/29/23 02:45, Bin Meng wrote:
> > On Thu, Jan 26, 2023 at 9:54 PM Daniel Henrique Barboza
> > <dbarboza@ventanamicro.com> wrote:
> >>
> >> As it is now, riscv_compute_fdt_addr() is receiving a dram_base, a
> >> mem_size (which is defaulted to MachineState::ram_size in all boards)
> >> and the FDT pointer. And it makes a very important assumption: the DRAM
> >> interval dram_base + mem_size is contiguous. This is indeed the case for
> >> most boards that uses a FDT.
> >
> > s/uses/use
> >
> >>
> >> The Icicle Kit board works with 2 distinct RAM banks that are separated
> >> by a gap. We have a lower bank with 1GiB size, a gap follows, then at
> >> 64GiB the high memory starts. MachineClass::default_ram_size for this
> >> board is set to 1.5Gb, and machine_init() is enforcing it as minimal RAM
> >> size, meaning that there we'll always have at least 512 MiB in the Hi
> >> RAM area.
> >>
> >> Using riscv_compute_fdt_addr() in this board is weird because not only
> >> the board has sparse RAM, and it's calling it using the base address of
> >> the Lo RAM area, but it's also using a mem_size that we have guarantees
> >> that it will go up to the Hi RAM. All the function assumptions doesn't
> >> work for this board.
> >>
> >> In fact, what makes the function works at all in this case is a
> >> coincidence.  Commit 1a475d39ef54 introduced a 3GB boundary for the FDT,
> >> down from 4Gb, that is enforced if dram_base is lower than 3072 MiB. For
> >> the Icicle Kit board, memmap[MICROCHIP_PFSOC_DRAM_LO].base is 0x80000000
> >> (2 Gb) and it has a 1Gb size, so it will fall in the conditions to put
> >> the FDT under a 3Gb address, which happens to be exactly at the end of
> >> DRAM_LO. If the base address of the Lo area started later than 3Gb this
> >> function would be unusable by the board. Changing any assumptions inside
> >> riscv_compute_fdt_addr() can also break it by accident as well.
> >>
> >> Let's change riscv_compute_fdt_addr() semantics to be appropriate to the
> >> Icicle Kit board and for future boards that might have sparse RAM
> >> topologies to worry about:
> >>
> >> - relieve the condition that the dram_base + mem_size area is contiguous,
> >> since this is already not the case today;
> >>
> >> - receive an extra 'dram_size' size attribute that refers to a contiguous
> >> RAM block that the board wants the FDT to reside on.
> >>
> >> Together with 'mem_size' and 'fdt', which are now now being consumed by a
> >> MachineState pointer, we're able to make clear assumptions based on the
> >> DRAM block and total mem_size available to ensure that the FDT will be put
> >> in a valid RAM address.
> >>
> >
> > Well written commit message. Thanks!
> >
> >> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> >> ---
> >>   hw/riscv/boot.c            | 38 ++++++++++++++++++++++++++------------
> >>   hw/riscv/microchip_pfsoc.c |  3 ++-
> >>   hw/riscv/sifive_u.c        |  3 ++-
> >>   hw/riscv/spike.c           |  3 ++-
> >>   hw/riscv/virt.c            |  3 ++-
> >>   include/hw/riscv/boot.h    |  4 ++--
> >>   6 files changed, 36 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/hw/riscv/boot.c b/hw/riscv/boot.c
> >> index a6f7b8ae8e..8f4991480b 100644
> >> --- a/hw/riscv/boot.c
> >> +++ b/hw/riscv/boot.c
> >> @@ -284,33 +284,47 @@ out:
> >>   }
> >>
> >>   /*
> >> - * The FDT should be put at the farthest point possible to
> >> - * avoid overwriting it with the kernel/initrd.
> >> + * This function makes an assumption that the DRAM interval
> >> + * 'dram_base' + 'dram_size' is contiguous.
> >>    *
> >> - * This function makes an assumption that the DRAM is
> >> - * contiguous. It also cares about 32-bit systems and
> >> - * will limit fdt_addr to be addressable by them even for
> >> - * 64-bit CPUs.
> >> + * Considering that 'dram_end' is the lowest value between
> >> + * the end of the DRAM block and MachineState->ram_size, the
> >> + * FDT location will vary according to 'dram_base':
> >> + *
> >> + * - if 'dram_base' is less that 3072 MiB, the FDT will be
> >> + * put at the lowest value between 3072 MiB and 'dram_end';
> >> + *
> >> + * - if 'dram_base' is higher than 3072 MiB, the FDT will be
> >> + * put at 'dram_end'.
> >>    *
> >>    * The FDT is fdt_packed() during the calculation.
> >>    */
> >> -uint32_t riscv_compute_fdt_addr(hwaddr dram_base, uint64_t mem_size,
> >> -                                void *fdt)
> >> +hwaddr riscv_compute_fdt_addr(hwaddr dram_base, hwaddr dram_size,
> >
> > Using hwaddr to represent a size looks weird. Although technically
> > they are the same ... I would leave this as it is.
>
> I'll leave it as it was back in patch 2 (uint64_t).
>
> >
> >> +                              MachineState *ms)
> >>   {
> >> -    uint64_t temp;
> >> -    hwaddr dram_end = dram_base + mem_size;
> >> -    int ret = fdt_pack(fdt);
> >> +    int ret = fdt_pack(ms->fdt);
> >> +    hwaddr dram_end, temp;
> >>       int fdtsize;
> >>
> >>       /* Should only fail if we've built a corrupted tree */
> >>       g_assert(ret == 0);
> >>
> >> -    fdtsize = fdt_totalsize(fdt);
> >> +    fdtsize = fdt_totalsize(ms->fdt);
> >>       if (fdtsize <= 0) {
> >>           error_report("invalid device-tree");
> >>           exit(1);
> >>       }
> >>
> >> +    /*
> >> +     * A dram_size == 0, usually from a MemMapEntry[].size element,
> >> +     * means that the DRAM block goes all the way to ms->ram_size.
> >> +     */
> >> +    if (dram_size == 0x0) {
> >> +        dram_end = dram_base + ms->ram_size;
> >> +    } else {
> >> +        dram_end = dram_base + MIN(ms->ram_size, dram_size);
> >> +    }
> >
> > How about:
> >
> > g_assert(dram_size < ms->ram_size);
>
> I don't believe that dram_size > ms->ram_size should be an error. A board can
> have a declared MemMapEntry.size that is larger than its current setting of
> ms->ram_size.

What use case is that? This updated function now has the assumption that:

1. dram_size being 0 meaning contiguous system RAM region from dram_base
2. otherwise dram_size being the *first* contiguous system RAM region
from dram_base

We can use g_assert(dram_size < ms->ram_size) to catch either case.

>
>
> > dram_end = dram_base + (dram_size ? dram_size : ms->ram_size);
>
> I can change the if/else statement up there for a ternary:
>
> dram_end = dram_base + (dram_size ? ms->ram_size : MIN(ms->ram_size, dram_size))
>

Regards,
Bin