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

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Message-ID: <YqjzXpzuBa4ATf9o@Asurada-Nvidia>
Date:   Tue, 14 Jun 2022 13:45:18 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
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Subject: Re: [PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match
 enforced cache coherency

Hi Kevin,

On Wed, Jun 08, 2022 at 11:48:27PM +0000, Tian, Kevin wrote:
> > > > 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.
> 
> It's unchanged per-domain but dynamic per-vm when multiple
> domains are added/removed (i.e. kvm->arch.noncoherent_dma_count).
> It's the latter being checked in the kvm.

I am going to send a v2, yet not quite getting the point here.
Meanwhile, Jason is on leave.

What, in your opinion, would be an accurate description here?

Thanks
Nic