linux-kernel - Re: [PATCH v4 05/22] iommu: introduce iommu invalidate API function

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Date:   Wed, 2 May 2018 10:31:50 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Jean Delvare <khali@...ux-fr.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...ux.intel.com>,
        "Liu@...rya.localdomain" <Liu@...rya.localdomain>
Subject: Re: [PATCH v4 05/22] iommu: introduce iommu invalidate API function

On 01/05/18 23:58, Jacob Pan wrote:
>>>> Maybe this should be called "NG_PAGE_PASID",  
>>> Sure. I was thinking page range already implies non-global pages.  
>>>> and "DOMAIN_PAGE" should
>>>> instead be "PAGE_PASID". If I understood their meaning correctly,
>>>> it would be more consistent with the rest.
>>>>  
>>> I am trying not to mix granu between request w/ PASID and w/o.
>>> DOMAIN_PAGE meant to be for request w/o PASID.  
>>
>> Is the distinction necessary? I understand the IOMMU side might offer
>> many possibilities for invalidation, but the user probably doesn't
>> need all of them. It might be easier to document, upstream and
>> maintain if we only specify what's currently needed by users (what
>> does QEMU VT-d use?) Others can always extend it by increasing the
>> version.
>>
>> Do you think that this invalidation message will be used outside of
>> BIND_PASID_TABLE context? I can't see an other use but who knows. At
>> the moment requests w/o PASID are managed with
>> VFIO_IOMMU_MAP/UNMAP_DMA, which doesn't require invalidation. And in
>> a BIND_PASID_TABLE context, IOMMUs requests w/o PASID are just a
>> special case using PASID 0 (for Arm and AMD) so I suppose they'll use
>> the same invalidation commands as requests w/ PASID.
>>
> My understanding is that for GIOVA use case, VT-d vIOMMU creates
> GIOVA-GPA mapping and the host shadows the 2nd level page tables to
> create GIOVA-HPA mapping. So when assigned device in the guest can do
> both DMA map/unmap and VFIO map/unmap, VFIO unmap is one time deal
> (I guess invalidation can be captured in other code path), but guest
> kernel use of DMA unmap could will trigger invalidation. QEMU needs to
> trap those invalidation and passdown to physical IOMMU. So we do need
> invalidation w/o PASID.

Hm, isn't this all done by host userspace? Whether guest does DMA
map/unmap or VFIO map/unmap, it creates/removes IOVA-GPA mappings in the
vIOMMU. QEMU captures invalidation requests for these mappings from the
guest, finds GPA-HVA in the shadow map and sends a VFIO map/unmap
request for IOVA-HVA.

Thanks,
Jean