linux-kernel - Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

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Message-ID: <20180604085742.GQ4251@umbus>
Date:   Mon, 4 Jun 2018 18:57:42 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        aik@...abs.ru, robh@...nel.org, joe@...ches.com,
        elfring@...rs.sourceforge.net, jasowang@...hat.com,
        mpe@...erman.id.au, hch@...radead.org
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for
 virito devices

On Thu, May 24, 2018 at 08:27:04AM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2018-05-23 at 21:50 +0300, Michael S. Tsirkin wrote:
> 
> > I re-read that discussion and I'm still unclear on the
> > original question, since I got several apparently
> > conflicting answers.
> > 
> > I asked:
> > 
> > 	Why isn't setting VIRTIO_F_IOMMU_PLATFORM on the
> > 	hypervisor side sufficient?
> 
> I thought I had replied to this...
> 
> There are a couple of reasons:
> 
> - First qemu doesn't know that the guest will switch to "secure mode"
> in advance. There is no difference between a normal and a secure
> partition until the partition does the magic UV call to "enter secure
> mode" and qemu doesn't see any of it. So who can set the flag here ?

This seems weird to me.  As a rule HV calls should go through qemu -
or be allowed to go directly to KVM *by* qemu.  We generally reserve
the latter for hot path things.  Since this isn't a hot path, having
the call handled directly by the kernel seems wrong.

Unless a "UV call" is something different I don't know about.

> - Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
> vhost) go through the emulated MMIO for every access to the guest,
> which adds additional overhead.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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