Re: [PATCH 2/2] acpi, apei: use appropriate pgprot_t to map GHES memory

[Zhang, Jonathan Zhixiong]

On 8/13/2015 4:14 AM, Will Deacon wrote:
> On Thu, Aug 13, 2015 at 10:24:32AM +0100, Matt Fleming wrote:
> > On Thu, 13 Aug, at 10:19:17AM, Ingo Molnar wrote:
> > > * Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > From: "Jonathan (Zhixiong) Zhang" <zjzhang@xxxxxxxxxxxxxx>
> > > >
> > > > With ACPI APEI firmware first handling, generic hardware error
> > > > record is updated by firmware in GHES memory region. On an arm64
> > > > platform, firmware updates GHES memory region with uncached
> > > > access attribute, and then Linux reads stale data from cache.
> > >
> > > So this paragraph does not parse for me ...
I will update the paragraph to explain that with current code,
PAGE_KERNEL is always used, this means that the kernel assumes
cache coherency to the memory region is maintained by firmware;
such assumption is not always correct.
> > > If it tries to explain a bug it falls very short of doing a proper job of that.
> >
> > I'll let Jonathan provide more details but I understood the problem to
> > be a cache (in)coherency issue.
> >
> > The kernel currently maps the the GHES memory region as cacheable
> > (PAGE_KERNEL) for all architectures. This memory region is used as a
> > communication buffer for reporting hardware errors from the firmware to
> > kernel. Essentially the firmware writes hardware error records there,
> > trigger an NMI/interrupt, and the GHES driver goes off and grabs the
> > error record from the GHES region.
> >
> > Since the firmware gets first crack at inspecting the error this
> > mechanism is referred to as "firmware first" in the ACPI spec.
> >
> > Now, there's a mismatch on arm64 platforms between how the kernel maps
> > the GHES region (PAGE_KERNEL) and how the firmware maps it
> > (EFI_MEMORY_UC, i.e. uncacheable), leading to the possibility of the
> > kernel GHES driver reading stale data from the cache when it receives
> > the NMI/interrupt.
> >
> > As for exactly why the arm64 firmware uses an uncached mapping, I
> > presume it's to avoid relying on the kernel to get the necessary cache
> > flushing correct.
> >
> > The proposed solution is to query the EFI memory map to ensure the
> > kernel uses a compatible mapping.
> >
> > None of this should affect x86, it still uses PAGE_KERNEL because we're
> > yet to see any hardware that has an EFI memory map entry for the GHES
> > region that's incompatible with PAGE_KERNEL.
> >
> > Jonathan, would you like to provide more details?
Not really. Matt and Will articulated it to the points.
> FWIW, that matches my understanding of the problem too. The ARM architecture
> refers to this situation as "mismatched memory attributes" and typically
> requires some explicit cache maintenance to achieve portable behaviour in
> this scenario.
>
> It's much better to avoid the mismatch in the first place, if you can,
> which is what this is all about.
>
> Will
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--
Jonathan (Zhixiong) Zhang
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
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