Re: Qemu v9.0.2: Boot failed qemu-arm with Linux next-20241017 tag.

["Alex Bennée" <alex.bennee@linaro.org>]

"Arnd Bergmann" <arnd@arndb.de> writes:

> On Sun, Oct 20, 2024, at 17:39, Naresh Kamboju wrote:
>> On Fri, 18 Oct 2024 at 12:35, Naresh Kamboju <naresh.kamboju@linaro.org> wrote:
>>>
>>> The QEMU-ARMv7 boot has failed with the Linux next-20241017 tag.
>>> The boot log is incomplete, and no kernel crash was detected.
>>> However, the system did not proceed far enough to reach the login prompt.
>>>
>
>> Anders bisected this boot regressions and found,
>> # first bad commit:
>>   [efe8419ae78d65e83edc31aad74b605c12e7d60c]
>>     vdso: Introduce vdso/page.h
>>
>> We are investigating the reason for boot failure due to this commit.
>
> Anders and I did the analysis on this, the problem turned out
> to be the early_init_dt_add_memory_arch() function in
> drivers/of/fdt.c, which does bitwise operations on PAGE_MASK
> with a 'u64' instead of phys_addr_t:
>
> void __init __weak early_init_dt_add_memory_arch(u64 base, u64 size)
> {
>         const u64 phys_offset = MIN_MEMBLOCK_ADDR;
>  
>         if (size < PAGE_SIZE - (base & ~PAGE_MASK)) {
>                 pr_warn("Ignoring memory block 0x%llx - 0x%llx\n",
>                         base, base + size);
>                 return;
>         }
>
>         if (!PAGE_ALIGNED(base)) {
>                 size -= PAGE_SIZE - (base & ~PAGE_MASK);
>                 base = PAGE_ALIGN(base);
>         }
>
> On non-LPAE arm32, this broke the existing behavior for
> large 32-bit memory sizes. The obvious fix is to change
> back the PAGE_MASK definition for 32-bit arm to a signed
> number.

Agreed. However I think we were masking a calling issue that:

    /* Actual RAM size depends on initial RAM and device memory settings */
    [VIRT_MEM] =                { GiB, LEGACY_RAMLIMIT_BYTES },

And:

  -m 4G

make no sense with no ARM_LPAE (which the kernel didn't have) but if you
pass -machine virt,gic-version=3,highmem=off (the default changed awhile
back) you will get a warning:

  qemu-system-arm: Addressing limited to 32 bits, but memory exceeds it by 1073741824 bytes

but I guess that didn't trigger for some reason before this patch?

> mips32, ppc32 and hexagon had the same definition as
> well, so I think we should change at least those in order
> to restore the previous behavior in case they are affected
> by the same bug (or a different one).
>
> x86-32 and arc git flipped the other way by the patch,
> from unsigned to signed, when CONFIG_ARC_HAS_PAE40
> or CONFIG_X86_PAE are set. I think we should keep
> the 'signed' behavior as this was a bugfix by itself,
> but we may want to change arc and x86-32 with short
> phys_addr_t the same way for consistency.
>
> On csky, m68k, microblaze, nios2, openrisc, parisc32,
> riscv32, sh, sparc32, um and xtensa, we've always used
> the 'unsigned' PAGE_MASK, and there is no 64-bit
> phys_addr_t, so I would lean towards staying with
> 'unsigned' in order to not introduce a regression.
> Alternatively we could choose to go with the 'signed'
> version on all 32-bit architectures unconditionally
> for consistency. Any preferences?
>
>       Arnd

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro