Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

From: David Gibson <david@gibson.dropbear.id.au>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mike Anderson <andmike@linux.ibm.com>,
	Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Alexey Kardashevskiy <aik@linux.ibm.com>,
	Ram Pai <linuxram@us.ibm.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Paul Mackerras <paulus@ozlabs.org>,
	iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
Date: Mon, 25 Mar 2019 11:57:10 +1100	[thread overview]
Message-ID: <20190325005710.GC29295@umbus> (raw)
In-Reply-To: <20190323165456-mutt-send-email-mst@kernel.org>

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On Sat, Mar 23, 2019 at 05:01:35PM -0400, Michael S. Tsirkin wrote:
> On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann wrote:
> > Michael S. Tsirkin <mst@redhat.com> writes:
[snip]
> > >> > Is there any justification to doing that beyond someone putting
> > >> > out slow code in the past?
> > >>
> > >> The definition of the ACCESS_PLATFORM flag is generic and captures the
> > >> notion of memory access restrictions for the device. Unfortunately, on
> > >> powerpc pSeries guests it also implies that the IOMMU is turned on
> > >
> > > IIUC that's really because on pSeries IOMMU is *always* turned on.
> > > Platform has no way to say what you want it to say
> > > which is bypass the iommu for the specific device.
> > 
> > Yes, that's correct. pSeries guests running on KVM are in a gray area
> > where theoretically they use an IOMMU but in practice KVM ignores it.
> > It's unfortunate but it's the reality on the ground today. :-/

Um.. I'm not sure what you mean by this.  As far as I'm concerned
there is always a guest-visible (paravirtualized) IOMMU, and that will
be backed onto the host IOMMU when necessary.

[Actually there is an IOMMU bypass hack that's used by the guest
 firmware, but I don't think we want to expose that]

> Well it's not just the reality, virt setups need something that
> emulated IOMMUs don't provide. That is not uncommon, e.g.
> intel's VTD has a "cache mode" field which AFAIK is only used for virt.

-- 
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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