linux-kernel - Re: [PATCH v2 2/3] soc: qcom: rmtfs: Support discarding guard pages

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Date:   Thu, 1 Jun 2023 14:26:12 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Bjorn Andersson <quic_bjorande@...cinc.com>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] soc: qcom: rmtfs: Support discarding guard pages

On Tue, May 30, 2023 at 04:36:42PM -0700, Bjorn Andersson wrote:
> In some configurations, the exact placement of the rmtfs shared memory
> region isn't so strict. The DeviceTree author can then choose to use the
> "size" property and rely on the OS for placement (in combination with
> "alloc-ranges", if desired).
> 
> But on some platforms the rmtfs memory region may not be allocated
> adjacent to regions allocated by other clients. Add support for
> discarding the first and last 4k block in the region, if
> qcom,use-guard-pages is specified in DeviceTree.
> 
> Signed-off-by: Bjorn Andersson <quic_bjorande@...cinc.com>
> ---
> 
> Changes since v1:
> - Drop the dma_alloc_coherent() based approach and just add support for
>   the guard pages.
> 
>  drivers/soc/qcom/rmtfs_mem.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/soc/qcom/rmtfs_mem.c b/drivers/soc/qcom/rmtfs_mem.c
> index f83811f51175..28238974d913 100644
> --- a/drivers/soc/qcom/rmtfs_mem.c
> +++ b/drivers/soc/qcom/rmtfs_mem.c
> @@ -213,6 +213,16 @@ static int qcom_rmtfs_mem_probe(struct platform_device *pdev)
>  		goto put_device;
>  	}
>  
> +	/*
> +	 * If requested, discard the first and last 4k block in order to ensure
> +	 * that the rmtfs region isn't adjacent to other protected regions.
> +	 */
> +	if (of_property_present(node, "qcom,use-guard-pages")) {
> +		rmtfs_mem->addr += SZ_4K;
> +		rmtfs_mem->base += SZ_4K;
> +		rmtfs_mem->size -= 2 * SZ_4K;
> +	}

It probably doesn't make a big difference in practice but I would say
there is no need to even memremap() the guard pages. If you adjust the
->addr and ->size before the memremap() then you don't need to modify
the ->base at all.

Thanks,
Stephan