linux-kernel - Re: [PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

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Message-ID: <20220608111724.GL1343366@nvidia.com>
Date:   Wed, 8 Jun 2022 08:17:24 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
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Subject: Re: [PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match
 enforced cache coherency

On Wed, Jun 08, 2022 at 08:28:03AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen
> > Sent: Monday, June 6, 2022 2:19 PM
> > 
> > From: Jason Gunthorpe <jgg@...dia.com>
> > 
> > The KVM mechanism for controlling wbinvd is only triggered during
> > kvm_vfio_group_add(), meaning it is a one-shot test done once the devices
> > are setup.
> 
> It's not one-shot. kvm_vfio_update_coherency() is called in both
> group_add() and group_del(). Then the coherency property is
> checked dynamically in wbinvd emulation:

>From the perspective of managing the domains that is still
one-shot. It doesn't get updated when individual devices are
added/removed to domains.

> given that I'm fine with the change in this patch. Even more probably
> we really want an explicit one-shot model so KVM can lock down
> the property once it starts to consume it then further adding a new
> group which would change the coherency is explicitly rejected and
> removing an existing group leaves it intact.

Why? Once wbinvd is enabled it is compatible with all domain
configurations, so just leave it on and ignore everything at that
point.

Jason