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

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Date: Wed, 27 Mar 2024 22:06:12 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
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

On 27.03.2024 10:04 PM, Volodymyr Babchuk wrote:
> 
> 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.

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.

Konrad