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

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Stephan,

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.

-- 
WBR, Volodymyr