[PATCH v1] qemu-arm: round down memory to multiple of 2MB

[Ard Biesheuvel]

On Thu, 11 Feb 2021 at 16:34, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> On 11.02.21 15:56, Ard Biesheuvel wrote:
> > On Thu, 11 Feb 2021 at 15:18, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >>
> >> On 11.02.21 13:04, Igor Opaniuk wrote:
> >>> From: Igor Opaniuk <igor.opaniuk at foundries.io>
> >>>
> >>> When LPAE is enabled, 1:1 mapping is created using 2 MB blocks.
> >>> In case amount of memory provided to QEMU is not multiple
> >>> of 2 MB, round down the amount of available memory to avoid hang
> >>> during MMU initialization.
> >>>
> >>> How to reproduce:
> >>> qemu-system-arm -machine virt -m 1058 -nographic -bios u-boot.bin - boots
> >>> qemu-system-arm -machine virt -m 1057 -nographic -bios u-boot.bin - hangs
> >>>
> >>> DRAM:  1 GiB
> >>> initcall: 60011df8
> >>> initcall: 60011904
> >>> New Stack Pointer is: 80fffe90
> >>> initcall: 60011a20
> >>> initcall: 60011bcc
> >>> initcall: 60011bd4
> >>> initcall: 600119b4
> >>> Relocation Offset is: 22042000
> >>> Relocating to 82042000, new gd at 81001ed0, sp at 80fffe90
> >>> initcall: 60011b8c
> >>> initcall: 82053ea0
> >>> initcall: 82053ea8
> >>> initcall: 60012040 (relocated to 82054040)
> >>> dram_bank_mmu_setup: bank: 0
> >>> --- hang here during mmu init ---
> >>>
> >>> Fixes: 3fa914af82("arm: qemu: implement enable_caches()")
> >>> Signed-off-by: Igor Opaniuk <igor.opaniuk at foundries.io>
> >>>
> >>> ---
> >>>
> >>>  board/emulation/qemu-arm/qemu-arm.c | 12 ++++++++++++
> >>>  1 file changed, 12 insertions(+)
> >>>
> >>> diff --git a/board/emulation/qemu-arm/qemu-arm.c b/board/emulation/qemu-arm/qemu-arm.c
> >>> index aa68bef469..841dd7af0e 100644
> >>> --- a/board/emulation/qemu-arm/qemu-arm.c
> >>> +++ b/board/emulation/qemu-arm/qemu-arm.c
> >>> @@ -84,6 +84,18 @@ int dram_init(void)
> >>>       if (fdtdec_setup_mem_size_base() != 0)
> >>>               return -EINVAL;
> >>>
> >>> +     /*
> >>> +      * When LPAE is enabled (ARMv7),
> >>> +      * 1:1 mapping is created using 2 MB blocks.
> >>> +      *
> >>> +      * In case amount of memory provided to QEMU
> >>> +      * is not multiple of 2 MB, round down the amount
> >>> +      * of available memory to avoid hang during MMU
> >>> +      * initialization.
> >>> +      */
> >>> +     if (CONFIG_IS_ENABLED(ARMV7_LPAE))
> >>> +             gd->ram_size -= (gd->ram_size % 0x200000);
> >>
> >> Is the problem LPAE specific?
> >> Couldn't you provoke same problem using an odd memory size without LPAE,
> >> e.g qemu-system-arm -m 536870908 (512 MiB - 4)?
> >>
> >
> > The above value means 512 GiB - 4 MiB, so that shouldn't be a problem.
> > I don't think QEMU's -m option takes fractional megabyte values.
> >
>
> $ qemu-system-arm -machine virt -cpu cortex-a15 -m 100005k \
> -bios denx/u-boot.bin -nographic
>
> => fdt addr $fdt_addr
> => fdt print /memory at 40000000
> memory at 40000000 {
>         reg = <0x00000000 0x40000000 0x00000000 0x061aa000>;
>         device_type = "memory";
> };
>
> Granularity seems to be 0x2000 = 8 KiB.
>

In that case, it seems easiest to me to always round down to the
nearest multiple of 2MB