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

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Message-ID: <87a5mjz8s3.fsf@epam.com>
Date: Wed, 27 Mar 2024 21:04:13 +0000
From: Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
To: Konrad Dybcio <konrad.dybcio@...aro.org>
CC: Caleb Connolly <caleb.connolly@...aro.org>,
        Bjorn Andersson
	<andersson@...nel.org>,
        "linux-arm-msm@...r.kernel.org"
	<linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Rob
 Clark <robdclark@...il.com>
Subject: Re: [PATCH] soc: qcom: cmd-db: map shared memory as WT, not WB


Hi Konrad,

Konrad Dybcio <konrad.dybcio@...aro.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@...m.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.

-- 
WBR, Volodymyr