From: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
To: Robin Murphy <robin.murphy@arm.com>,
Marijn Suijten <marijn.suijten@somainline.org>,
Will Deacon <will@kernel.org>
Cc: Konrad Dybcio <konrad.dybcio@somainline.org>,
~postmarketos/upstreaming@lists.sr.ht,
linux-arm-msm@vger.kernel.org, bjorn.andersson@linaro.org,
linux-arm-kernel@lists.infradead.org,
iommu@lists.linux-foundation.org, martin.botka@somainline.org,
angelogioacchino.delregno@somainline.org,
jamipkettunen@somainline.org, Rob Clark <robdclark@gmail.com>,
Joerg Roedel <joro@8bytes.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] iommu/qcom: Write TCR before TTBRs to fix ASID access behavior
Date: Wed, 8 Jun 2022 13:03:13 +0200 [thread overview]
Message-ID: <24931967-87be-5207-eb2b-47c064aee0c7@collabora.com> (raw)
In-Reply-To: <4911a6c8-e494-5a5e-015d-d9fcf886d253@arm.com>
Il 08/06/22 12:54, Robin Murphy ha scritto:
> On 2022-06-08 11:27, AngeloGioacchino Del Regno wrote:
>> Il 06/06/22 00:06, Marijn Suijten ha scritto:
>>> On 2022-05-31 16:55:59, Will Deacon wrote:
>>>> On Fri, May 27, 2022 at 11:28:57PM +0200, Konrad Dybcio wrote:
>>>>> From: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
>>>>>
>>>>> As also stated in the arm-smmu driver, we must write the TCR before
>>>>> writing the TTBRs, since the TCR determines the access behavior of
>>>>> some fields.
>>>>
>>>> Where is this stated in the arm-smmu driver?
>>>>
>>>>>
>>>>> Signed-off-by: AngeloGioacchino Del Regno
>>>>> <angelogioacchino.delregno@somainline.org>
>>>>> Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
>>>>> Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
>>>>> ---
>>>>> drivers/iommu/arm/arm-smmu/qcom_iommu.c | 12 ++++++------
>>>>> 1 file changed, 6 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/drivers/iommu/arm/arm-smmu/qcom_iommu.c
>>>>> b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
>>>>> index 1728d4d7fe25..75f353866c40 100644
>>>>> --- a/drivers/iommu/arm/arm-smmu/qcom_iommu.c
>>>>> +++ b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
>>>>> @@ -273,18 +273,18 @@ static int qcom_iommu_init_domain(struct iommu_domain
>>>>> *domain,
>>>>> ctx->secure_init = true;
>>>>> }
>>>>> - /* TTBRs */
>>>>> - iommu_writeq(ctx, ARM_SMMU_CB_TTBR0,
>>>>> - pgtbl_cfg.arm_lpae_s1_cfg.ttbr |
>>>>> - FIELD_PREP(ARM_SMMU_TTBRn_ASID, ctx->asid));
>>>>> - iommu_writeq(ctx, ARM_SMMU_CB_TTBR1, 0);
>>>>> -
>>>>> /* TCR */
>>>>> iommu_writel(ctx, ARM_SMMU_CB_TCR2,
>>>>> arm_smmu_lpae_tcr2(&pgtbl_cfg));
>>>>> iommu_writel(ctx, ARM_SMMU_CB_TCR,
>>>>> arm_smmu_lpae_tcr(&pgtbl_cfg) | ARM_SMMU_TCR_EAE);
>>>>> + /* TTBRs */
>>>>> + iommu_writeq(ctx, ARM_SMMU_CB_TTBR0,
>>>>> + pgtbl_cfg.arm_lpae_s1_cfg.ttbr |
>>>>> + FIELD_PREP(ARM_SMMU_TTBRn_ASID, ctx->asid));
>>>>> + iommu_writeq(ctx, ARM_SMMU_CB_TTBR1, 0);
>>>>
>>>> I'd have thought that SCTLR.M would be clear here, so it shouldn't matter
>>>> what order we write these in.
>>>
>>> Having tested the series without this particular patch on 8976 (Sony
>>> Loire Suzu), it doesn't seem to matter indeed. I'll ask around if this
>>> "access behaviour" was observed on a different board/platform.
>>>
>>> - Marijn
>>
>> On some platforms, the bootloader (and/or the hypervisor) is performing some
>> initialization of the IOMMU which, depending on the actual firmware version
>> that ran before booting Linux, may or may not leave SCTLR.M cleared.
>
> But does it actually matter even then? If we're only allowed to program the same
> ASID that was in use beforehand, then logically we can't be changing TCR2.AS in a
> way that makes any difference anyway.
>
> I see no point in pretending to worry about theoretical architectural correctness
> in a driver tied to specific implementations that already violate the given
> architecture in many other ways. If there's a known firmware implementation that
> definitely requires this, that should be called out; otherwise, there doesn't seem
> much justification for the patch at all.
>
This is something I wrote more than one year ago, hence I don't remember clearly,
but if my memories aren't failing me, this was necessary to enable support for
the AArch64 pagetables.
If that doesn't make sense to you, I guess that Marijn or Konrad can help testing
switching to AA64 PT with the incorrect programming sequence.
Aside from that, as a strictly personal opinion (and nothing else), I think that
ensuring architectural correctness *where possible* can only be good: I don't see
why we should intentionally keep a wrong programming sequence in principle.
Regards,
Angelo
> Thanks,
> Robin.
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next prev parent reply other threads:[~2022-06-08 11:04 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-27 21:28 [PATCH 0/6] Fix and extend Qualcomm IOMMU support Konrad Dybcio
2022-05-27 21:28 ` [PATCH 1/6] iommu/qcom: Use the asid read from device-tree if specified Konrad Dybcio
2022-05-31 15:46 ` Will Deacon
2022-05-31 16:15 ` Rob Clark
2022-05-31 16:19 ` Will Deacon
2022-05-31 20:57 ` Rob Clark
2022-06-03 18:03 ` Konrad Dybcio
2022-06-08 10:25 ` AngeloGioacchino Del Regno
2022-05-27 21:28 ` [PATCH 2/6] iommu/qcom: Write TCR before TTBRs to fix ASID access behavior Konrad Dybcio
2022-05-31 15:55 ` Will Deacon
2022-05-31 16:26 ` Robin Murphy
2022-06-05 22:06 ` Marijn Suijten
2022-06-08 10:27 ` AngeloGioacchino Del Regno
2022-06-08 10:54 ` Robin Murphy
2022-06-08 11:03 ` AngeloGioacchino Del Regno [this message]
2022-05-27 21:28 ` [PATCH 3/6] iommu/qcom: Properly reset the IOMMU context Konrad Dybcio
2022-05-27 21:28 ` [PATCH 4/6] iommu/qcom: Add support for AArch64 IOMMU pagetables Konrad Dybcio
2022-05-28 2:03 ` kernel test robot
2022-06-02 14:17 ` Rob Herring
2022-05-27 21:29 ` [PATCH 5/6] iommu/qcom: Index contexts by asid number to allow asid 0 Konrad Dybcio
2022-06-03 15:14 ` Brian Masney
2022-05-27 21:29 ` [PATCH 6/6] iommu/qcom: Add support for QCIOMMUv2 and QCIOMMU-500 secured contexts Konrad Dybcio
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