Re: Bug report: kernel paniced when system hibernates

Anup Patel
2023-05-18T06:54:18Z

On Wed, May 17, 2023 at 8:26 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>
> On Wed, May 17, 2023 at 1:28 PM Conor Dooley <conor.dooley@microchip.com> wrote:
> >
> > Hey Alex,
> >
> > On Wed, May 17, 2023 at 10:58:02AM +0200, Alexandre Ghiti wrote:
> > > On Tue, May 16, 2023 at 1:12 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
> >
> > > > On Tue, May 16, 2023 at 11:24 AM Song Shuai <suagrfillet@gmail.com> wrote:
> > > > I actually removed this flag a few years ago, and I have to admit that
> > > > I need to check if that's necessary: the goal of commit 3335068f8721
> > > > ("riscv: Use PUD/P4D/PGD pages for the linear mapping") is to expose
> > > > the "right" start of DRAM so that we can align virtual and physical
> > > > addresses on a 1GB boundary.
> > > >
> > > > So I have to check if a nomap region is actually added as a
> > > > memblock.memory.regions[] or not: if yes, that's perfect, let's add
> > > > the nomap attributes to the PMP regions, otherwise, I don't think that
> > > > is a good solution.
> > >
> > > So here is the current linear mapping without nomap in openSBI:
> > >
> > > ---[ Linear mapping ]---
> > > 0xff60000000000000-0xff60000000200000 0x0000000080000000 2M
> > > PMD D A G . . W R V
> > > 0xff60000000200000-0xff60000000e00000 0x0000000080200000 12M
> > > PMD D A G . . . R V
> > >
> > > And below the linear mapping with nomap in openSBI:
> > >
> > > ---[ Linear mapping ]---
> > > 0xff60000000080000-0xff60000000200000 0x0000000080080000 1536K
> > > PTE D A G . . W R V
> > > 0xff60000000200000-0xff60000000e00000 0x0000000080200000 12M
> > > PMD D A G . . . R V
> > >
> > > So adding nomap does not misalign virtual and physical addresses, it
> > > prevents the usage of 1GB page for this area though, so that's a
> > > solution, we just lose this 1GB page here.
> > >
> > > But even though that may be the fix, I think we also need to fix that
> > > in the kernel as it would break compatibility with certain versions of
> > > openSBI *if* we fix openSBI...So here are a few solutions:
> > >
> > > 1. we can mark all "mmode_resv" nodes in the device tree as nomap,
> > > before the linear mapping is established (IIUC, those nodes are added
> > > by openSBI to advertise PMP regions)
> > > -> This amounts to the same fix as opensbi and we lose the 1GB hugepage.
> >
> > AFAIU, losing the 1 GB hugepage is a regression, which would make this
> > not an option, right?
>
> Not sure this is a real regression, I'd rather avoid it, but as
> mentioned in my first answer, Mike Rapoport showed that it was making
> no difference performance-wise...
>
> >
> > > 2. we can tweak pfn_is_nosave function to *not* save pfn corresponding
> > > to PMP regions
> > > -> We don't lose the 1GB hugepage \o/
> > > 3. we can use register_nosave_region() to not save the "mmode_resv"
> > > regions (x86 does that
> > > https://elixir.bootlin.com/linux/v6.4-rc1/source/arch/x86/kernel/e820.c#L753)
> > > -> We don't lose the 1GB hugepage \o/
> > > 4. Given JeeHeng pointer to
> > > https://elixir.bootlin.com/linux/v6.4-rc1/source/kernel/power/snapshot.c#L1340,
> > > we can mark those pages as non-readable and make the hibernation
> > > process not save those pages
> > > -> Very late-in-the-day idea, not sure what it's worth, we also
> > > lose the 1GB hugepage...
> >
> > Ditto here re: introducing another regression.
> >
> > > To me, the best solution is 3 as it would prepare for other similar
> > > issues later, it is similar to x86 and it allows us to keep 1GB
> > > hugepages.
> > >
> > > I have been thinking, and to me nomap does not provide anything since
> > > the kernel should not address this memory range, so if it does, we
> > > must fix the kernel.
> > >
> > > Let me know what you all think, I'll be preparing a PoC of 3 in the meantime!
> >
> > #3 would probably get my vote too. It seems like you could use it
> > dynamically if there was to be a future other provider of "mmode_resv"
> > regions, rather than doing something location-specific.
> >
> > We should probably document these opensbi reserved memory nodes though
> > in a dt-binding or w/e if we are going to be relying on them to not
> > crash!

Depending on a particular node name is fragile. If we really need
information from DT then I suggest adding "no-save-restore" DT
property in reserved memory nodes.

>
> Yes, you're right, let's see what Atish and Anup think!

I think we have two possible approaches:

1) Update OpenSBI to set "no-map" DT property for firmware
reserved regions. We were doing this previously but removed
it later for performance reasons mentioned by Alex. It is also
worth mentioning that ARM Trusted Firmware also sets "no-map"
DT property for firmware reserved regions.

2) Add a new "no-save-restore" DT property in the reserved
memory DT bindings. The hibernate support of Linux arch/riscv
will use this DT property to exclude memory regions from
save-restore. The EFI implementation of EDK2 and U-Boot
should do the following:
1) Treat all memory having "no-map" DT property as EFI
reserved memory
2) Treat all memory not having "no-map" DT property and
not having "no-save-restore" DT property as EfiBootServicesData
3) Treat all memory not having "no-map" DT property and
having "no-save-restore" DT property as EfiRuntimeServiceData
(Refer,
https://devicetree-specification.readthedocs.io/en/latest/chapter3-devicenodes.html#reserved-memory-and-uefi)

Personally, I am leaning towards approach#1 since approach#2
will require changing DeviceTree specification as well.

Regards,
Anup

>
> Thanks for your quick answers Conor and Song, really appreciated!
>
> Alex
>
> >
> > Thanks for working on this,
> > Conor.
> >