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Date:   Fri, 11 Aug 2023 09:45:18 -0700
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
CC:     Jason Gunthorpe <jgg@nvidia.com>, "Liu, Yi L" <yi.l.liu@intel.com>,
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Subject: Re: [PATCH v4 09/12] iommu/vt-d: Add iotlb flush for nested domain
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On Fri, Aug 11, 2023 at 03:52:52AM +0000, Tian, Kevin wrote:
 
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Friday, August 11, 2023 5:03 AM
> >
> > > > > Is there a use case for invaliation only SW emulated rings, and do we
> > > > > care about optimizing for the wrap around case?
> > > >
> > > > Hmm, why a SW emulated ring?
> > >
> > > That is what you are building. The VMM catches the write of the
> > > producer pointer and the VMM SW bundles it up to call into the kernel.
> >
> > Still not fully getting it. Do you mean a ring that is prepared
> > by the VMM? I think the only case that we need to handle a ring
> > is what I did by forwarding the guest CMDQ (a ring) to the host
> > directly. Not sure why VMM would need another ring for those
> > linearized invalidation commands. Or maybe I misunderstood..
> >
> 
> iiuc the point of a ring-based native format is to maximum code reuse
> when later in-kernel fast invalidation path (from kvm to smmu driver)
> is enabled. Then both slow (via vmm) and fast (in-kernel) path use
> the same logic to handle guest invalidation queue.

I see. That's about the fast path topic. Thanks for the input.

> But if stepping back a bit supporting an array-based non-native format
> could simplify the uAPI design and allows code sharing for array among
> vendor drivers. You can still keep the entry as native format then the
> only difference with future in-kernel fast path is just on walking an array
> vs. walking a ring. And VMM doesn't need to expose non-invalidate
> cmds to the kernel and then be skipped.

Ah, so we might still design the uAPI to be ring based at this
moment, yet don't support a case CONS > 0 to leave that to an
upgrade in the future.

I will try estimating a bit how complicated to implement the
ring, to see if we could just start with that. Otherwise, will
just start with an array.

Thanks
Nic