linux-kernel - Re: [RFC PATCH 01/11] iommu/arm-smmu-v3: Add feature detection for HTTU

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Date:   Sun, 7 Feb 2021 09:42:49 +0800
From:   Keqian Zhu <zhukeqian1@...wei.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
CC:     Robin Murphy <robin.murphy@....com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>, <iommu@...ts.linux-foundation.org>,
        "Will Deacon" <will@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Marc Zyngier" <maz@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Mark Rutland" <mark.rutland@....com>, <jiangkunkun@...wei.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Cornelia Huck <cohuck@...hat.com>, <lushenming@...wei.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        James Morse <james.morse@....com>,
        <wanghaibin.wang@...wei.com>, "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RFC PATCH 01/11] iommu/arm-smmu-v3: Add feature detection for
 HTTU

Hi Jean,

On 2021/2/5 17:51, Jean-Philippe Brucker wrote:
> Hi Keqian,
> 
> On Fri, Feb 05, 2021 at 05:13:50PM +0800, Keqian Zhu wrote:
>>> We need to accommodate the firmware override as well if we need this to be meaningful. Jean-Philippe is already carrying a suitable patch in the SVA stack[1].
>> Robin, Thanks for pointing it out.
>>
>> Jean, I see that the IORT HTTU flag overrides the hardware register info unconditionally. I have some concern about it:
>>
>> If the override flag has HTTU but hardware doesn't support it, then driver will use this feature but receive access fault or permission fault from SMMU unexpectedly.
>> 1) If IOPF is not supported, then kernel can not work normally.
>> 2) If IOPF is supported, kernel will perform useless actions, such as HTTU based dma dirty tracking (this series).
>>
>> As the IORT spec doesn't give an explicit explanation for HTTU override, can we comprehend it as a mask for HTTU related hardware register?
> 
> To me "Overrides the value of SMMU_IDR0.HTTU" is clear enough: disregard
> the value of SMMU_IDR0.HTTU and use the one specified by IORT instead. And
> that's both ways, since there is no validity mask for the IORT value: if
> there is an IORT table, always ignore SMMU_IDR0.HTTU.
> 
> That's how the SMMU driver implements the COHACC bit, which has the same
> wording in IORT. So I think we should implement HTTU the same way.
OK, and Robin said that the latest IORT spec literally states it.

> 
> One complication is that there is no equivalent override for device tree.
> I think it can be added later if necessary, because unlike IORT it can be
> tri state (property not present, overriden positive, overridden negative).
Yeah, that would be more flexible. ;-)

> 
> Thanks,
> Jean
> 
> .
> 
Thanks,
Keqian