Re: [PATCH V4 0/8] virtio: Solution to restrict memory access under Xen using xen-grant DMA-mapping layer

Viresh Kumar
2022-06-15T06:24:14Z

Hi Oleksandr,

On Mon, Jun 6, 2022 at 10:16 AM Oleksandr Tyshchenko
<olekstysh@gmail.com> wrote:
> The high level idea is to create new Xen’s grant table based DMA-mapping layer for the guest Linux whose main
> purpose is to provide a special 64-bit DMA address which is formed by using the grant reference (for a page
> to be shared with the backend) with offset and setting the highest address bit (this is for the backend to
> be able to distinguish grant ref based DMA address from normal GPA). For this to work we need the ability
> to allocate contiguous (consecutive) grant references for multi-page allocations. And the backend then needs
> to offer VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 feature bits (it must support virtio-mmio modern
> transport for 64-bit addresses in the virtqueue).

I was trying your series, from Linus's tree now and started seeing
boot failures,
failed to mount rootfs. And the reason probably is these messages:

[ 1.222498] virtio_scsi virtio1: device must provide VIRTIO_F_ACCESS_PLATFORM
[ 1.316334] virtio_net virtio0: device must provide VIRTIO_F_ACCESS_PLATFORM

I understand from your email that the backends need to offer
VIRTIO_F_ACCESS_PLATFORM flag now, but should this requirement be a
bit soft ? I mean shouldn't we allow both types of backends to run with the same
kernel, ones that offer this feature and others that don't ? The ones that don't
offer the feature, should continue to work like they used to, i.e.
without the restricted
memory access feature.

I am testing Xen currently with help of Qemu over my x86 desktop and
these backends
(scsi and net) are part of QEMU itself I think, and I don't really
want to go and make the
change there.

Thanks.

--
Viresh