Re: [PATCH] soc: qcom: cmd-db: map shared memory as WT, not WB

[Stephan Gerhold <stephan@gerhold.net>]

On Wed, Apr 10, 2024 at 10:12:37PM +0000, Volodymyr Babchuk wrote:
> Stephan Gerhold <stephan@gerhold.net> writes:
> > On Wed, Mar 27, 2024 at 11:29:09PM +0000, Caleb Connolly wrote:
> >> On 27/03/2024 21:06, Konrad Dybcio wrote:
> >> > On 27.03.2024 10:04 PM, Volodymyr Babchuk wrote:
> >> >> Konrad Dybcio <konrad.dybcio@linaro.org> writes:
> >> >>> On 27.03.2024 9:09 PM, Volodymyr Babchuk wrote:
> >> >>>> It appears that hardware does not like cacheable accesses to this
> >> >>>> region. Trying to access this shared memory region as Normal Memory
> >> >>>> leads to secure interrupt which causes an endless loop somewhere in
> >> >>>> Trust Zone.
> >> >>>>
> >> >>>> The only reason it is working right now is because Qualcomm Hypervisor
> >> >>>> maps the same region as Non-Cacheable memory in Stage 2 translation
> >> >>>> tables. The issue manifests if we want to use another hypervisor (like
> >> >>>> Xen or KVM), which does not know anything about those specific
> >> >>>> mappings. This patch fixes the issue by mapping the shared memory as
> >> >>>> Write-Through. This removes dependency on correct mappings in Stage 2
> >> >>>> tables.
> >> >>>>
> >> >>>> I tested this on SA8155P with Xen.
> >> >>>>
> >> >>>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> >> >>>> ---
> >> >>>
> >> >>> Interesting..
> >> >>>
> >> >>> +Doug, Rob have you ever seen this on Chrome? (FYI, Volodymyr, chromebooks
> >> >>> ship with no qcom hypervisor)
> >> >>
> >> >> Well, maybe I was wrong when called this thing "hypervisor". All I know
> >> >> that it sits in hyp.mbn partition and all what it does is setup EL2
> >> >> before switching to EL1 and running UEFI.
> >> >>
> >> >> In my experiments I replaced contents of hyp.mbn with U-Boot, which gave
> >> >> me access to EL2 and I was able to boot Xen and then Linux as Dom0.
> >> > 
> >> > Yeah we're talking about the same thing. I was just curious whether
> >> > the Chrome folks have heard of it, or whether they have any changes/
> >> > workarounds for it.
> >> 
> >> Does Linux ever write to this region? Given that the Chromebooks don't
> >> seem to have issues with this (we have a bunch of them in pmOS and I'd
> >> be very very surprised if this was an issue there which nobody had tried
> >> upstreaming before) I'd guess the significant difference here is between
> >> booting Linux in EL2 (as Chromebooks do?) vs with Xen.
> >> 
> >
> > FWIW: This old patch series from Stephen Boyd is closely related:
> > https://urldefense.com/v3/__https://lore.kernel.org/linux-arm-msm/20190910160903.65694-1-swboyd@chromium.org/__;!!GF_29dbcQIUBPA!yGecMHGezwkDU9t7XATVTI80PNGjZdQV2xsYFTl6EhpMMsRf_7xryKx8mEVpmTwTcKMGaaWomtyvr05zFcmsf2Kk$
> > [lore[.]kernel[.]org]
> >
> >> The main use case I have is to map the command-db memory region on
> >> Qualcomm devices with a read-only mapping. It's already a const marked
> >> pointer and the API returns const pointers as well, so this series
> >> makes sure that even stray writes can't modify the memory.
> >
> > Stephen, what was the end result of that patch series? Mapping the
> > cmd-db read-only sounds cleaner than trying to be lucky with the right
> > set of cache flags.
> >
> 
> I checked the series, but I am afraid that I have no capacity to finish
> this. Will it be okay to move forward with my patch? I understand that
> this is not the best solution, but it is simple and it works. If this is
> fine, I'll send v2 with all comments addressed.
> 

My current understanding is that the important property here is to have
a non-cacheable mapping, which is the case for both MEMREMAP_WT and
MEMREMAP_WC, but not MEMREMAP_WB. Unfortunately, the MEMREMAP_RO option
Stephen introduced is also a cacheable mapping, which still seems to
trigger the issue in some cases. I'm not sure why a cache writeback
still happens when the mapping is read-only and nobody writes anything.

You can also test it if you want. For a quick test,

-	cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WB);
+	cmd_db_header = ioremap_prot(rmem->base, rmem->size, _PAGE_KERNEL_RO);

should be (largely) equivalent to MEMREMAP_RO with Stephen's patch
series. I asked Nikita to test this on SC7180 and it still seems to
cause the crash.

It seems to work only with a read-only non-cacheable mapping, e.g. with

+	cmd_db_header = ioremap_prot(rmem->base, rmem->size,
				     ((PROT_NORMAL_NC & ~PTE_WRITE) | PTE_RDONLY));

The lines I just suggested for testing are highly architecture-specific
though so not usable for a proper patch. If MEMREMAP_RO does not solve
the real problem here then the work to make an usable read-only mapping
would go beyond just finishing Stephen's patch series, since one would
need to introduce some kind of MEMREMAP_RO_NC flag that creates a
read-only non-cacheable mapping.

It is definitely easier to just change the driver to use the existing
MEMREMAP_WC. Given the crash you found, the hardware/firmware seems to
have a built-in write protection on most platforms anyway. :D

Thanks,
Stephan