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

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Message-ID: <0a61c07d-edfe-2738-380d-33d39e40fc0a@amd.com>
Date:   Thu, 17 Jan 2019 08:44:36 +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 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