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




On 15.06.22 09:23, Viresh Kumar wrote:
Hi Oleksandr,


Hello Viresh



On Mon, Jun 6, 2022 at 10:16 AM Oleksandr Tyshchenko
<olekstysh@xxxxxxxxx> 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.


Thank you for testing on x86.


I assume your guest type in HVM. Within current series the PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS is set for *all* type of Xen guests if CONFIG_XEN_VIRTIO is enabled.

I have to admit that from the very beginning it could be possible to configure for PV and HVM guests separately [1] because the usage of grant mappings for virtio is mandatory for paravirtualized guest, but not strictly necessary for the fully virtualized guests (if the backends are in Dom0). But it was decided to drop these extra options (including XEN_HVM_VIRTIO_GRANT) and leave only single one CONFIG_XEN_VIRTIO.

I see that Juergen has already pushed a fix.

Sorry for the inconvenience.



[1] https://lore.kernel.org/xen-devel/1649963973-22879-3-git-send-email-olekstysh@xxxxxxxxx/



Thanks.

--
Viresh

--
Regards,

Oleksandr Tyshchenko




 


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