From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted Date: Sun, 11 Aug 2019 04:12:50 -0400 Message-ID: <20190811041035-mutt-send-email-mst@kernel.org> References: <87zhrj8kcp.fsf@morokweng.localdomain> <20190810143038-mutt-send-email-mst@kernel.org> <20190810220702.GA5964@ram.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20190810220702.GA5964@ram.ibm.com> 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: Ram Pai Cc: Benjamin Herrenschmidt , Alexey Kardashevskiy , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Paul Mackerras , iommu@lists.linux-foundation.org, linuxppc-devel@lists.ozlabs.org, Christoph Hellwig , David Gibson List-Id: virtualization@lists.linuxfoundation.org On Sat, Aug 10, 2019 at 03:07:02PM -0700, Ram Pai wrote: > On Sat, Aug 10, 2019 at 02:57:17PM -0400, Michael S. Tsirkin wrote: > > On Tue, Jan 29, 2019 at 03:08:12PM -0200, Thiago Jung Bauermann wrote: > > > > > > Hello, > > > > > > With Christoph's rework of the DMA API that recently landed, the patch > > > below is the only change needed in virtio to make it work in a POWER > > > secure guest under the ultravisor. > > > > > > The other change we need (making sure the device's dma_map_ops is NULL > > > so that the dma-direct/swiotlb code is used) can be made in > > > powerpc-specific code. > > > > > > Of course, I also have patches (soon to be posted as RFC) which hook up > > > to the powerpc secure guest support code. > > > > > > What do you think? > > > > > > >From d0629a36a75c678b4a72b853f8f7f8c17eedd6b3 Mon Sep 17 00:00:00 2001 > > > From: Thiago Jung Bauermann > > > Date: Thu, 24 Jan 2019 22:08:02 -0200 > > > Subject: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted > > > > > > The host can't access the guest memory when it's encrypted, so using > > > regular memory pages for the ring isn't an option. Go through the DMA API. > > > > > > Signed-off-by: Thiago Jung Bauermann > > > --- > > > drivers/virtio/virtio_ring.c | 5 ++++- > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > index cd7e755484e3..321a27075380 100644 > > > --- a/drivers/virtio/virtio_ring.c > > > +++ b/drivers/virtio/virtio_ring.c > > > @@ -259,8 +259,11 @@ static bool vring_use_dma_api(struct virtio_device *vdev) > > > * not work without an even larger kludge. Instead, enable > > > * the DMA API if we're a Xen guest, which at least allows > > > * all of the sensible Xen configurations to work correctly. > > > + * > > > + * Also, if guest memory is encrypted the host can't access > > > + * it directly. In this case, we'll need to use the DMA API. > > > */ > > > - if (xen_domain()) > > > + if (xen_domain() || sev_active()) > > > return true; > > > > > > return false; > > > > So I gave this lots of thought, and I'm coming round to > > basically accepting something very similar to this patch. > > > > But not exactly like this :). > > > > Let's see what are the requirements. > > > > If > > > > 1. We do not trust the device (so we want to use a bounce buffer with it) > > 2. DMA address is also a physical address of a buffer > > > > then we should use DMA API with virtio. > > > > > > sev_active() above is one way to put (1). I can't say I love it but > > it's tolerable. > > > > > > But we also want promise from DMA API about 2. > > > > > > Without promise 2 we simply can't use DMA API with a legacy device. > > > > > > Otherwise, on a SEV system with an IOMMU which isn't 1:1 > > and with a virtio device without ACCESS_PLATFORM, we are trying > > to pass a virtual address, and devices without ACCESS_PLATFORM > > can only access CPU physical addresses. > > > > So something like: > > > > dma_addr_is_phys_addr? > > > On our Secure pseries platform, dma address is physical address and this > proposal will help us, use DMA API. > > On our normal pseries platform, dma address is physical address too. > But we do not necessarily need to use the DMA API. We can use the DMA > API, but our handlers will do the same thing, the generic virtio handlers > would do. If there is an opt-out option; even when dma addr is same as > physical addr, than there will be less code duplication. > > Would something like this be better. > > (dma_addr_is_phys_addr && arch_want_to_use_dma_api()) ? > > > RP I think sev_active() is an OK replacement for arch_want_to_use_dma_api. So just the addition of dma_addr_is_phys_addr would be enough. > > > -- > > MST > > -- > Ram Pai