From: "Michael S. Tsirkin" <mst@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>,
Ram Pai <linuxram@us.ibm.com>,
robh@kernel.org, aik@ozlabs.ru, jasowang@redhat.com,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org, joe@perches.com,
linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net,
david@gibson.dropbear.id.au, cohuck@redhat.com,
pawel.moll@arm.com, Tom Lendacky <thomas.lendacky@amd.com>,
"Rustad, Mark D" <mark.d.rustad@intel.com>
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices
Date: Wed, 13 Jun 2018 16:49:39 +0300 [thread overview]
Message-ID: <20180613161806-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20180608063655.GA32080@infradead.org>
On Thu, Jun 07, 2018 at 11:36:55PM -0700, Christoph Hellwig wrote:
> > This seems to be what was being asked for in this thread,
> > with comments claiming IOMMU flag adds too much overhead.
>
> Right now it means implementing a virtual iommu, which I agree is
> way too much overhead.
Well not really. The flag in question will have a desired effect without
a virtual iommu.
> > SEV guys report that they just set the iommu flag and then it all works.
> > I guess if there's translation we can think of this as a kind of iommu.
> > Maybe we should rename PLATFORM_IOMMU to PLARTFORM_TRANSLATION?
>
> VIRTIO_F_BEHAVES_LIKE_A_REAL_PCI_DEVICE_DONT_TRY_TO_OUTSMART_ME
>
> as said it's not just translations, it is cache coherence as well.
Well it's only for DMA. So maybe PLATFORM_DMA.
I suspect people will then come and complain that they
do *not* want cache coherence tricks because virtio is
running on a CPU, but we'll see.
> > And apparently some people complain that just setting that flag makes
> > qemu check translation on each access with an unacceptable performance
> > overhead. Forcing same behaviour for everyone on general principles
> > even without the flag is unlikely to make them happy.
>
> That sounds like a qemu implementation bug. If qemu knowns that
> guest physiscall == guest dma space there is no need to check.
Possibly. Or it's possible it's all just theoretical, no one
posted any numbers.
--
MST
next prev parent reply other threads:[~2018-06-13 13:49 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-22 6:33 [RFC V2] virtio: Add platform specific DMA API translation for virito devices Anshuman Khandual
2018-05-23 18:50 ` Michael S. Tsirkin
2018-05-23 22:27 ` Benjamin Herrenschmidt
2018-05-24 7:17 ` Christoph Hellwig
2018-05-25 17:45 ` Michael S. Tsirkin
2018-05-28 23:48 ` Benjamin Herrenschmidt
2018-05-28 23:56 ` Benjamin Herrenschmidt
2018-05-29 14:03 ` Christoph Hellwig
2018-05-29 22:13 ` Benjamin Herrenschmidt
2018-06-04 8:57 ` David Gibson
2018-06-04 9:48 ` Benjamin Herrenschmidt
2018-06-04 12:50 ` Michael S. Tsirkin
2018-06-05 1:52 ` David Gibson
2018-06-04 12:43 ` Michael S. Tsirkin
2018-06-04 12:55 ` Christoph Hellwig
2018-06-04 13:14 ` Benjamin Herrenschmidt
2018-06-04 16:34 ` Michael S. Tsirkin
2018-06-04 13:11 ` Benjamin Herrenschmidt
2018-06-04 16:21 ` Michael S. Tsirkin
2018-06-04 23:26 ` Benjamin Herrenschmidt
2018-06-05 1:25 ` Michael S. Tsirkin
2018-06-05 4:52 ` Christoph Hellwig
2018-05-24 7:21 ` Ram Pai
2018-05-31 3:39 ` Anshuman Khandual
2018-05-31 17:43 ` Michael S. Tsirkin
2018-06-07 5:23 ` Christoph Hellwig
2018-06-07 16:28 ` Michael S. Tsirkin
2018-06-08 6:36 ` Christoph Hellwig
2018-06-13 13:49 ` Michael S. Tsirkin [this message]
2018-06-11 2:39 ` Ram Pai
2018-06-11 3:28 ` Michael S. Tsirkin
2018-06-11 3:34 ` Benjamin Herrenschmidt
2018-06-13 14:23 ` Michael S. Tsirkin
2018-06-11 3:29 ` Benjamin Herrenschmidt
2018-06-13 7:41 ` Christoph Hellwig
2018-06-13 12:25 ` Benjamin Herrenschmidt
2018-06-13 13:11 ` Benjamin Herrenschmidt
2018-06-15 9:16 ` Christoph Hellwig
2018-06-16 1:07 ` Benjamin Herrenschmidt
2018-06-13 13:59 ` Michael S. Tsirkin
2018-06-13 14:03 ` Michael S. Tsirkin
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