linux-kernel - Re: [PATCH v2 02/10] iommu: Introduce a new iommu_group_replace

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Date:   Mon, 13 Feb 2023 16:34:32 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: Re: [PATCH v2 02/10] iommu: Introduce a new
 iommu_group_replace_domain() API

On 2023/2/13 10:24, Tian, Kevin wrote:
>> From: Jason Gunthorpe<jgg@...dia.com>
>> Sent: Saturday, February 11, 2023 8:45 AM
>>
>> On Fri, Feb 10, 2023 at 04:51:10PM -0700, Alex Williamson wrote:
>>> On Tue, 7 Feb 2023 13:17:54 -0800
>>> Nicolin Chen<nicolinc@...dia.com>  wrote:
>>>
>>>> qemu has a need to replace the translations associated with a domain
>>>> when the guest does large-scale operations like switching between an
>>>> IDENTITY domain and, say, dma-iommu.c.
>>>>
>>>> Currently, it does this by replacing all the mappings in a single
>>>> domain, but this is very inefficient and means that domains have to be
>>>> per-device rather than per-translation.
>>>>
>>>> Provide a high-level API to allow replacements of one domain with
>>>> another. This is similar to a detach/attach cycle except it doesn't
>>>> force the group to go to the blocking domain in-between.
>>>>
>>>> By removing this forced blocking domain the iommu driver has the
>>>> opportunity to implement an atomic replacement of the domains to the
>>>> greatest extent its hardware allows.
>>>>
>>>> It could be possible to adderss this by simply removing the protection
>>>> from the iommu_attach_group(), but it is not so clear if that is safe
>>>> for the few users. Thus, add a new API to serve this new purpose.
>>>>
>>>> Atomic replacement allows the qemu emulation of the viommu to be
>> more
>>>> complete, as real hardware has this ability.
>>> I was under the impression that we could not atomically switch a
>>> device's domain relative to in-flight DMA.
> it's possible as long as the mappings for in-flight DMA don't change
> in the transition.
> 

It also requires the mappings in old and new domains are identical. In
another word, any IOVA should be translated to a same result no matter
through the old domain, the new domain, or hardware caches.

A similar replacement has been implemented in the code. For example,
the Intel IOMMU driver dynamically transforms a large range (2M or 1G)
mapping from discrete 4k pages to a super pages to improve the paging
cache efficiency.

Best regards,
baolu