linux-kernel - Re: [PATCH] iommu/amd: Fix IOMMU page flush when detach all devices from a domain

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Date:   Thu, 17 Jan 2019 11:48:44 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     "joro@...tes.org" <joro@...tes.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "Singh, Brijesh" <brijesh.singh@....com>
Subject: Re: [PATCH] iommu/amd: Fix IOMMU page flush when detach all devices
 from a domain

Joerg,

On 1/17/19 3:44 PM, Suravee Suthikulpanit wrote:
> Joerg,
> 
> On 1/17/19 12:08 AM, joro@...tes.org wrote:
>> On Wed, Jan 16, 2019 at 02:08:55PM +0000, Suthikulpanit, Suravee wrote:
>>> Actually, I am not sure how we would be missing the flush on the last device.
>>> In my test, I am seeing the flush command being issued correctly during
>>> vfio_unmap_unpin(), which is after all devices are detached.
>>> Although, I might be missing your point here. Could you please elaborate?
>>
>> Okay, you are right, the patch effectivly adds an unconditional flush of
>> the domain on all iommus when the last device is detached. So it is
>> correct in that regard. But that code path is also used in the
>> iommu_unmap() path.
>>
>> The problem now is, that with your change we send flush commands to all
>> IOMMUs in the unmap path when no device is attached to the domain.
>> This will hurt performance there, no?
>>
>> Regards,
>>
>>     Joerg
>>
> 
> Sounds like we need a way to track state of each IOMMU for a domain.
> What if we define the following:
> 
>    enum IOMMU_DOMAIN_STATES {
>      DOMAIN_FREE = -1,
>      DOMAIN_DETACHED = 0,
>      DOMAIN_ATTACHED >= 1
>    }
> 
> We should be able to use the dev_iommu[] to help track the state.
>      - In amd_iommu_domain_alloc, we initialize the array to DOMAIN_FREE
>      - In do_attach(), we change to DOMAIN_ATTACH or we can increment the count
>        if it is already in DOMAIN_ATTACH state.
>      - In do_detach(). we change to DOMAIN_DETACH.
> 
> Then, in __domain_flush_pages, we issue command when the dev_iommu[] >= 0.
> This should preserve previous behavior, and only add flushing condition to
> the specific IOMMU in detached state. Please let me know what you think.
> 
> Regards,
> Suravee

By the way, I just sent V2 of this patch since it might be more clear.

Regards,
Suravee