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

[Stephan Gerhold <stephan@gerhold.net>]

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://lore.kernel.org/linux-arm-msm/20190910160903.65694-1-swboyd@chromium.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.

Thanks,
Stephan