linux-kernel - Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep

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Message-ID: <20221114141109.GG30263@willie-the-truck>
Date:   Mon, 14 Nov 2022 14:11:10 +0000
From:   Will Deacon <will@...nel.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     catalin.marinas@....com, robin.murphy@....com,
        amit.pundir@...aro.org, andersson@...nel.org,
        quic_sibis@...cinc.com, sumit.semwal@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from
 arch_dma_prep_coherent()"

On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote:
> This reverts commit c44094eee32f32f175aadc0efcac449d99b1bbf7.
> 
> As reported by Amit [1], dropping cache invalidation from
> arch_dma_prep_coherent() triggers a crash on the Qualcomm SM8250 platform
> (most probably on other Qcom platforms too). The reason is, Qcom
> qcom_q6v5_mss driver copies the firmware metadata and shares it with modem
> for validation. The modem has a secure block (XPU) that will trigger a
> whole system crash if the shared memory is accessed by the CPU while modem
> is poking at it.
> 
> To avoid this issue, the qcom_q6v5_mss driver allocates a chunk of memory
> with no kernel mapping, vmap's it, copies the firmware metadata and
> unvmap's it. Finally the address is then shared with modem for metadata
> validation [2].
> 
> Now because of the removal of cache invalidation from
> arch_dma_prep_coherent(), there will be cache lines associated with this
> memory even after sharing with modem. So when the CPU accesses it, the XPU
> violation gets triggered.

This last past is a non-sequitur: the buffer is no longer mapped on the CPU
side, so how would the CPU access it?

As I just replied to Amit, we need more information about what this
"access" is and how it is being detected.

Will