From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices Date: Thu, 24 May 2018 08:27:04 +1000 Message-ID: References: <20180522063317.20956-1-khandual@linux.vnet.ibm.com> <20180523213703-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180523213703-mutt-send-email-mst@kernel.org> 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: "Michael S. Tsirkin" , Anshuman Khandual Cc: robh@kernel.org, mpe@ellerman.id.au, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, hch@infradead.org, joe@perches.com, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, david@gibson.dropbear.id.au List-Id: virtualization@lists.linuxfoundation.org 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 ? - 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. Cheers, Ben. > > > > arch/powerpc/include/asm/dma-mapping.h | 6 ++++++ > > arch/powerpc/platforms/pseries/iommu.c | 11 +++++++++++ > > drivers/virtio/virtio_ring.c | 10 ++++++++++ > > 3 files changed, 27 insertions(+) > > > > diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h > > index 8fa3945..056e578 100644 > > --- a/arch/powerpc/include/asm/dma-mapping.h > > +++ b/arch/powerpc/include/asm/dma-mapping.h > > @@ -115,4 +115,10 @@ extern u64 __dma_get_required_mask(struct device *dev); > > #define ARCH_HAS_DMA_MMAP_COHERENT > > > > #endif /* __KERNEL__ */ > > + > > +#define platform_forces_virtio_dma platform_forces_virtio_dma > > + > > +struct virtio_device; > > + > > +extern bool platform_forces_virtio_dma(struct virtio_device *vdev); > > #endif /* _ASM_DMA_MAPPING_H */ > > diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c > > index 06f0296..a2ec15a 100644 > > --- a/arch/powerpc/platforms/pseries/iommu.c > > +++ b/arch/powerpc/platforms/pseries/iommu.c > > @@ -38,6 +38,7 @@ > > #include > > #include > > #include > > +#include > > #include > > #include > > #include > > @@ -1396,3 +1397,13 @@ static int __init disable_multitce(char *str) > > __setup("multitce=", disable_multitce); > > > > machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init); > > + > > +bool platform_forces_virtio_dma(struct virtio_device *vdev) > > +{ > > + /* > > + * On protected guest platforms, force virtio core to use DMA > > + * MAP API for all virtio devices. But there can also be some > > + * exceptions for individual devices like virtio balloon. > > + */ > > + return (of_find_compatible_node(NULL, NULL, "ibm,ultravisor") != NULL); > > +} > > Isn't this kind of slow? vring_use_dma_api is on > data path and supposed to be very fast. > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > index 21d464a..47ea6c3 100644 > > --- a/drivers/virtio/virtio_ring.c > > +++ b/drivers/virtio/virtio_ring.c > > @@ -141,8 +141,18 @@ struct vring_virtqueue { > > * unconditionally on data path. > > */ > > > > +#ifndef platform_forces_virtio_dma > > +static inline bool platform_forces_virtio_dma(struct virtio_device *vdev) > > +{ > > + return false; > > +} > > +#endif > > + > > static bool vring_use_dma_api(struct virtio_device *vdev) > > { > > + if (platform_forces_virtio_dma(vdev)) > > + return true; > > + > > if (!virtio_has_iommu_quirk(vdev)) > > return true; > > > > -- > > 2.9.3