Re: [PATCH 2/6] iommu/qcom: Write TCR before TTBRs to fix ASID access behavior

AngeloGioacchino Del Regno
2022-06-08T11:03:26Z

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.