linux-kernel - Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET

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Date:   Thu, 30 Sep 2021 21:43:58 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        "Lu, Baolu" <baolu.lu@...el.com>
Cc:     baolu.lu@...ux.intel.com,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

On 2021/9/30 16:49, Tian, Kevin wrote:
>> From: Jason Gunthorpe <jgg@...dia.com>
>> Sent: Thursday, September 23, 2021 8:22 PM
>>
>>>> These are different things and need different bits. Since the ARM path
>>>> has a lot more code supporting it, I'd suggest Intel should change
>>>> their code to use IOMMU_BLOCK_NO_SNOOP and abandon
>> IOMMU_CACHE.
>>>
>>> I didn't fully get this point. The end result is same, i.e. making the DMA
>>> cache-coherent when IOMMU_CACHE is set. Or if you help define the
>>> behavior of IOMMU_CACHE, what will you define now?
>>
>> It is clearly specifying how the kernel API works:
>>
>>   !IOMMU_CACHE
>>     must call arch cache flushers
>>   IOMMU_CACHE -
>>     do not call arch cache flushers
>>   IOMMU_CACHE|IOMMU_BLOCK_NO_SNOOP -
>>     dot not arch cache flushers, and ignore the no snoop bit.
> 
> Who will set IOMMU_BLOCK_NO_SNOOP? I feel this is arch specific
> knowledge about how cache coherency is implemented, i.e.
> when IOMMU_CACHE is set intel-iommu driver just maps it to
> blocking no-snoop. It's not necessarily to be an attribute in
> the same level as IOMMU_CACHE?
> 
>>
>> On Intel it should refuse to create a !IOMMU_CACHE since the HW can't
>> do that.
> 
> Agree. In reality I guess this is not hit because all devices are marked
> coherent on Intel platforms...
> 
> Baolu, any insight here?

I am trying to follow the discussion here. Please guide me if I didn't
get the right context.

Here, we are discussing arch_sync_dma_for_cpu() and
arch_sync_dma_for_device(). The x86 arch has clflush to sync dma buffer
for device, but I can't see any instruction to sync dma buffer for cpu
if the device is not cache coherent. Is that the reason why x86 can't
have an implementation for arch_sync_dma_for_cpu(), hence all devices
are marked coherent?

> Thanks
> Kevin
> 

Best regards,
baolu