From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Date: Wed, 8 Aug 2018 23:31:46 +0300 Message-ID: <20180808232210-mutt-send-email-mst__20574.9060879045$1533760216$gmane$org@kernel.org> References: <20180806094243.GA16032@infradead.org> <6c707d6d33ac25a42265c2e9b521c2416d72c739.camel@kernel.crashing.org> <20180807062117.GD32709@infradead.org> <20180807135505.GA29034@infradead.org> <2103ecfe52d23cec03f185d08a87bfad9c9d82b5.camel@kernel.crashing.org> <20180808063158.GA2474@infradead.org> <4b596883892b5cb5560bef26fcd249e7107173ac.camel@kernel.crashing.org> <20180808123036.GA2525@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Benjamin Herrenschmidt Cc: robh@kernel.org, srikar@linux.vnet.ibm.com, mpe@ellerman.id.au, Will Deacon , linux-kernel@vger.kernel.org, linuxram@us.ibm.com, virtualization@lists.linux-foundation.org, Christoph Hellwig , jean-philippe.brucker@arm.com, paulus@samba.org, marc.zyngier@arm.com, joe@perches.com, robin.murphy@arm.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com, Anshuman Khandual List-Id: virtualization@lists.linuxfoundation.org On Wed, Aug 08, 2018 at 11:18:13PM +1000, Benjamin Herrenschmidt wrote: > Sure, but all of this is just the configuration of the iommu. But I > think we agree here, and your point remains valid, indeed my proposed > hack: > > > if ((flags & VIRTIO_F_IOMMU_PLATFORM) || arch_virtio_wants_dma_ops()) > > Will only work if the IOMMU and non-IOMMU path are completely equivalent. > > We can provide that guarantee for our secure VM case, but not generally so if > we were to go down the route of a quirk in virtio, it might be better to > make it painfully obvious that it's specific to that one case with a different > kind of turd: > > - if (xen_domain()) > + if (xen_domain() || pseries_secure_vm()) > return true; I don't think it's pseries specific actually. E.g. I suspect AMD SEV might benefit from the same kind of hack. > So to summarize, and make sure I'm not missing something, the two approaches > at hand are either: > > 1- The above, which is a one liner and contained in the guest, so that's nice, but > also means another turd in virtio which isn't ... > > 2- We force pseries to always set VIRTIO_F_IOMMU_PLATFORM, but with the current > architecture on our side that will force virtio to always go through an emulated > iommu, as pseries doesn't have the concept of a real bypass window, and thus will > impact performance for both secure and non-secure VMs. > > 3- Invent a property that can be put in selected PCI device tree nodes that > indicates that for that device specifically, the iommu can be bypassed, along with > a hypercall to turn that bypass on/off. Virtio would then use VIRTIO_F_IOMMU_PLATFORM > but its DT nodes would also have that property and Linux would notice it and turn > bypass on. For completeness, virtio could also have its own bounce buffer outside of DMA API one. I don't see lots of benefits to this though. > The resulting properties of those options are: > > 1- Is what I want because it's the simplest, provides the best performance now, > and works without code changes to qemu or non-secure Linux. However it does > add a tiny turd to virtio which is annoying. > > 2- This works but it puts the iommu in the way always, thus reducing virtio performance > accross the board for pseries unless we only do that for secure VMs but that is > difficult (as discussed earlier). > > 3- This would recover the performance lost in -2-, however it requires qemu *and* > guest changes. Specifically, existing guests (RHEL 7 etc...) would get the > performance hit of -2- unless modified to call that 'enable bypass' call, which > isn't great. > > So imho we have to chose one of 3 not-great solutions here... Unless I missed > something in your ideas of course. > > Cheers, > Ben. > >