linux-kernel - RE: [PATCH v2 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

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Date:   Thu, 16 Jun 2022 06:29:22 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
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Subject: RE: [PATCH v2 2/5] vfio/iommu_type1: Prefer to reuse domains vs match
 enforced cache coherency

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Thursday, June 16, 2022 8:03 AM
> 
> From: Jason Gunthorpe <jgg@...dia.com>
> 
> The KVM mechanism for controlling wbinvd is based on OR of the coherency
> property of all devices attached to a guest, no matter those devices are
> attached to a single domain or multiple domains.
> 
> So, there is no value in trying to push a device that could do enforced
> cache coherency to a dedicated domain vs re-using an existing domain
> which is non-coherent since KVM won't be able to take advantage of it.
> This just wastes domain memory.
> 
> Simplify this code and eliminate the test. This removes the only logic
> that needed to have a dummy domain attached prior to searching for a
> matching domain and simplifies the next patches.
> 
> It's unclear whether we want to further optimize the Intel driver to
> update the domain coherency after a device is detached from it, at
> least not before KVM can be verified to handle such dynamics in related
> emulation paths (wbinvd, vcpu load, write_cr0, ept, etc.). In reality
> we don't see an usage requiring such optimization as the only device
> which imposes such non-coherency is Intel GPU which even doesn't
> support hotplug/hot remove.
> 
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>