Re: RISC-V reserved memory problems

[Conor Dooley]

On Thu, Mar 09, 2023 at 11:30:07AM +0100, Alexandre Ghiti wrote:
> Hi Conor,

Hey Alex!

> On 8/16/22 22:41, Conor.Dooley@xxxxxxxxxxxxx wrote:
> > Hey all,
> > We've run into a bit of a problem with reserved memory on PolarFire, or
> > more accurately a pair of problems that seem to have opposite fixes.
> > 
> > The first of these problems is triggered when trying to implement a
> > remoteproc driver. To get the reserved memory buffer, remoteproc
> > does an of_reserved_mem_lookup(), something like:
> > 
> > 	np = of_parse_phandle(pdev->of_node, "memory-region", 0);
> > 	if (!np)
> > 		return -EINVAL;
> > 
> > 	rmem = of_reserved_mem_lookup(np);
> > 	if (!rmem)
> > 		return -EINVAL;
> > 
> > of_reserved_mem_lookup() then uses reserved_mem[i].name to try and find
> > a match - but this was triggering kernel panics for us. We did some
> > debugging and found that the name string's pointer was pointing to an
> > address in the 0x4000_0000 range. The minimum reproduction for this
> 
> 
> 0x4000_0000 corresponds to DTB_EARLY_BASE_VA: this is the address that is
> used to map the dtb before we can access it using the linear mapping.
> 
> 
> > crash is attached - it hacks in some print_reserved_mem()s into
> > setup_vm_final() around a tlb flush so you can see the before/after.
> > (You'll need a reserved memory node in your dts to replicate)
> > 
> > The output is like so, with the same crash as in the remoteproc driver:
> > 
> > [    0.000000] Linux version 6.0.0-rc1-00001-g0d9d6953d834 (conor@wendy) (riscv64-unknown-linux-gnu-gcc (g5964b5cd727) 11.1.0, GNU ld (GNU Binutils) 2.37) #1 SMP Tue Aug 16 13:42:09 IST 2022
> > [    0.000000] OF: fdt: Ignoring memory range 0x80000000 - 0x80200000
> > [    0.000000] Machine model: Microchip PolarFire-SoC Icicle Kit
> > [    0.000000] earlycon: ns16550a0 at MMIO32 0x0000000020100000 (options '115200n8')
> > [    0.000000] printk: bootconsole [ns16550a0] enabled
> > [    0.000000] printk: debug: skip boot console de-registration.
> > [    0.000000] efi: UEFI not found.
> > [    0.000000] before flush
> > [    0.000000] OF: reserved mem: debug name is fabricbuf@ae000000
> > [    0.000000] after flush
> > [    0.000000] Unable to handle kernel paging request at virtual address 00000000401c31ac
> 
> 
> You take the trap here because the mapping for the dtb does not exist in
> swapper_pg_dir, but you don't need this mapping anymore as you can access
> the device tree through the linear mapping now.
> 
> I would say that: you build your kernel with CONFIG_BUILTIN_DTB

I do not have any "NONPORTABLE" options selected.

> and then you
> don't call early_init_dt_verify which resets initial_boot_params to the
> linear mapping address (it was initially set to 0x4000_0000 in parse_dtb).
> If that's the case, does the following fix your issue?

I dunno, I don't hit this issue anymore as of 50e63dd8ed92 ("riscv: fix
reserved memory setup"). That might not be the right fix in the grand
scheme of things, but it solved problems that both I and others were
hitting.

> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 376d2827e736..2b09f0bd8432 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -276,6 +276,7 @@ void __init setup_arch(char **cmdline_p)
>         efi_init();
>         paging_init();
>  #if IS_ENABLED(CONFIG_BUILTIN_DTB)
> +       initial_boot_params = __va(XIP_FIXUP(dtb_early_pa));
>         unflatten_and_copy_device_tree();
>  #else
>         if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))

I'll try and allocate some time for looking at this next week, got some
items on my todo list first. I doubt it'll help me cos I have not get
that option set, but I can go and test reserved memory stuff for systems
that do use it (I have a k210).

> > [    0.000000] Oops [#1]
> > [    0.000000] Modules linked in:
> > [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.0.0-rc1-00001-g0d9d6953d834 #1
> > [    0.000000] Hardware name: Microchip PolarFire-SoC Icicle Kit (DT)
> > [    0.000000] epc : string+0x4a/0xea
> > [    0.000000]  ra : vsnprintf+0x1e4/0x336
> > [    0.000000] epc : ffffffff80335ea0 ra : ffffffff80338936 sp : ffffffff81203be0
> > [    0.000000]  gp : ffffffff812e0a98 tp : ffffffff8120de40 t0 : 0000000000000000
> > [    0.000000]  t1 : ffffffff81203e28 t2 : 7265736572203a46 s0 : ffffffff81203c20
> > [    0.000000]  s1 : ffffffff81203e28 a0 : ffffffff81203d22 a1 : 0000000000000000
> > [    0.000000]  a2 : ffffffff81203d08 a3 : 0000000081203d21 a4 : ffffffffffffffff
> > [    0.000000]  a5 : 00000000401c31ac a6 : ffff0a00ffffff04 a7 : ffffffffffffffff
> > [    0.000000]  s2 : ffffffff81203d08 s3 : ffffffff81203d00 s4 : 0000000000000008
> > [    0.000000]  s5 : ffffffff000000ff s6 : 0000000000ffffff s7 : 00000000ffffff00
> > [    0.000000]  s8 : ffffffff80d9821a s9 : ffffffff81203d22 s10: 0000000000000002
> > [    0.000000]  s11: ffffffff80d9821c t3 : ffffffff812f3617 t4 : ffffffff812f3617
> > [    0.000000]  t5 : ffffffff812f3618 t6 : ffffffff81203d08
> > [    0.000000] status: 0000000200000100 badaddr: 00000000401c31ac cause: 000000000000000d
> > [    0.000000] [<ffffffff80338936>] vsnprintf+0x1e4/0x336
> > [    0.000000] [<ffffffff80055ae2>] vprintk_store+0xf6/0x344
> > [    0.000000] [<ffffffff80055d86>] vprintk_emit+0x56/0x192
> > [    0.000000] [<ffffffff80055ed8>] vprintk_default+0x16/0x1e
> > [    0.000000] [<ffffffff800563d2>] vprintk+0x72/0x80
> > [    0.000000] [<ffffffff806813b2>] _printk+0x36/0x50
> > [    0.000000] [<ffffffff8068af48>] print_reserved_mem+0x1c/0x24
> > [    0.000000] [<ffffffff808057ec>] paging_init+0x528/0x5bc
> > [    0.000000] [<ffffffff808031ae>] setup_arch+0xd0/0x592
> > [    0.000000] [<ffffffff8080070e>] start_kernel+0x82/0x73c
> > [    0.000000] ---[ end trace 0000000000000000 ]---
> > [    0.000000] Kernel panic - not syncing: Attempted to kill the idle task!
> > [    0.000000] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---
> > 
> > We traced this back to early_init_fdt_scan_reserved_mem() in
> > setup_bootmem() - moving it later back up the boot sequence to
> > after the dt has been remapped etc has fixed the problem for us.
> > 
> > The least movement to get it working is attached, and also pushed
> > here: git.kernel.org/conor/c/1735589baefc
> > 
> > The second problem is a bit more complicated to explain - but we
> > found the solution conflicted with the remoteproc fix as we had
> > to move early_init_fdt_scan_reserved_mem() _earlier_ in the boot
> > process to solve this one.
> > 
> > We want to have a node in our devicetree that contains some memory
> > that is non-cached & marked as reserved-memory. Maybe we have just
> > missed something, but from what we've seen:
> > - the really early setup looks at the dtb, picks the highest bit
> >     of memory and puts the dtb etc there so it can start using it
> > - early_init_fdt_scan_reserved_mem() is then called, which figures
> >     out if memory is reserved or not.
> > 
> > Unfortunately, the highest bit of memory is the non-cached bit so
> > everything falls over, but we can avoid this by moving the call to
> > early_init_fdt_scan_reserved_mem() above the dtb memblock alloc that
> > takes place right before it in setup_bootmem().
> 
> 
> And then I suppose the allocations you are mentioning happen in
> unflatten_XXX, so parsing the device tree for reserved memory nodes before
> this should do the trick. Does the following fix your second issue?

Again, I'll have to look at this.
To give you a quick answer, I believe it is the create_pgd_mapping()
stuff in setup_vm_final() actually.
I'm not super sure about this particular bit of the issue, but in the
email I sent more recently that expands on these issues (that Mike
replied to the other day) the similar issue that I saw the allocations
in reserved memory were inside paging_init().

> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 2b09f0bd8432..94b3d049fe9d 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -277,14 +277,15 @@ void __init setup_arch(char **cmdline_p)
>         paging_init();
>  #if IS_ENABLED(CONFIG_BUILTIN_DTB)
>         initial_boot_params = __va(XIP_FIXUP(dtb_early_pa));
> +       early_init_fdt_scan_reserved_mem();
>         unflatten_and_copy_device_tree();
>  #else
> -       if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> +       if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa)))) {
> +               early_init_fdt_scan_reserved_mem();
>                 unflatten_device_tree();

Previously, this was problematic (doing the scan before unflattening).
I moved the scan to the earliest possible place that I could call it
without issues.
I'll get back to you...

> -       else
> +       } else
>                 pr_err("No DTB found in kernel mappings\n");
>  #endif
> -       early_init_fdt_scan_reserved_mem();
>         misc_mem_init();
> 
>         init_resources();
> 
> 
> 
> > 
> > Obviously, both of these changes are moving the function call in
> > opposite directions and we can only really do one of them. We are not
> > sure if what we are doing with the non-cached reserved-memory section
> > is just not permitted & cannot work - or if this is something that
> > was overlooked for RISC-V specifically and works for other archs.
> > 
> > It does seem like the first issue is a real bug, and I am happy to
> > submit the patch for that whenever - but having two problems with
> > opposite fixes seemed as if there was something else lurking that we
> > just don't have enough understanding to detect.
> > 
> > Any help would be great!
> > 
> > Thanks,
> > Conor.
> > 
> 
> Even if that does not fix your issue, the first patch is necessary as it
> fixes initial_boot_params.

Sure, send a patch (with a nice commit message please, so that I don't
have to remember the whole process to review it!!) as my brain has long
since swapped out the nitty gritty of the unflattening -> scanning
process.

Thanks Alex!

Conor.

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