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

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Date:   Wed, 17 Apr 2019 18:42:44 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ram Pai <linuxram@...ibm.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Michael Roth <mdroth@...ux.vnet.ibm.com>,
        Mike Anderson <andmike@...ux.ibm.com>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted


David Gibson <david@...son.dropbear.id.au> writes:

> 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@...hat.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.

There is, but vhost will ignore it and directly map the guest memory
when ACCESS_PLATFORM (the flag previously known as IOMMU_PLATFORM) isn't
set. From QEMU's hw/virtio/vhost.c:

static int vhost_dev_has_iommu(struct vhost_dev *dev)
{
    VirtIODevice *vdev = dev->vdev;

    return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
}

static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
                              hwaddr *plen, int is_write)
{
    if (!vhost_dev_has_iommu(dev)) {
        return cpu_physical_memory_map(addr, plen, is_write);
    } else {
        return (void *)(uintptr_t)addr;
    }
}

--
Thiago Jung Bauermann
IBM Linux Technology Center