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

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Message-ID: <19bb6ff0-04ff-4e88-8c8a-499c054bdea4@quicinc.com>
Date: Thu, 28 Mar 2024 17:31:04 +0530
From: "Maulik Shah (mkshah)" <quic_mkshah@...cinc.com>
To: Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
CC: Caleb Connolly <caleb.connolly@...aro.org>, <Volodymyr_Babchuk@...m.com>,
        "andersson@...nel.org >> Bjorn Andersson" <andersson@...nel.org>,
        Konrad
 Dybcio <konrad.dybcio@...aro.org>,
        linux-arm-msm
	<linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soc: qcom: cmd-db: map shared memory as WT, not WB



On 3/28/2024 1:39 AM, 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.

Linux does not write into cmd-db region. This region is write protected 
by XPU. Making this region uncached magically solves the XPU write fault
issue.

Can you please include above details?

> 
> 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.

Using MEMREMAP_WC also resolves for qcm6490, see below comment.

> 
> I tested this on SA8155P with Xen.
> 
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@...m.com>
> ---
>   drivers/soc/qcom/cmd-db.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> index a5fd68411bed5..dd5ababdb476c 100644
> --- a/drivers/soc/qcom/cmd-db.c
> +++ b/drivers/soc/qcom/cmd-db.c
> @@ -324,7 +324,7 @@ static int cmd_db_dev_probe(struct platform_device *pdev)
>   		return -EINVAL;
>   	}
>   
> -	cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WB);
> +	cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WT);

In downstream, we have below which resolved similar issue on qcm6490.

cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WC);

Downstream SA8155P also have MEMREMAP_WC. Can you please give it a try 
on your device?

Thanks,
Maulik