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Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> writes:

> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Date: Sat, 28 Nov 2020 22:33:51 +0200
> Subject: [PATCH V3 00/23] IOREQ feature (+ virtio-mmio) on Arm
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> Hello all.
> The purpose of this patch series is to add IOREQ/DM support to Xen on Arm.
> You can find an initial discussion at [1] and RFC/V1/V2 series at [2]/[3]/[4].
> Xen on Arm requires some implementation to forward guest MMIO access to a 
> device
> model in order to implement virtio-mmio backend or even mediator outside of 
> hypervisor.
> As Xen on x86 already contains required support this series tries to make it 
> common
> and introduce Arm specific bits plus some new functionality. Patch series is 
> based on
> Julien's PoC "xen/arm: Add support for Guest IO forwarding to a device 
> emulator".
> Besides splitting existing IOREQ/DM support and introducing Arm side, the 
> series
> also includes virtio-mmio related changes (last 2 patches for toolstack)
> for the reviewers to be able to see how the whole picture could look
> like.

Thanks for posting the latest version.

> According to the initial discussion there are a few open questions/concerns
> regarding security, performance in VirtIO solution:
> 1. virtio-mmio vs virtio-pci, SPI vs MSI, different use-cases require 
> different
>    transport...

I think I'm repeating things here I've said in various ephemeral video
chats over the last few weeks but I should probably put things down on
the record.

I think the original intention of the virtio framers is advanced
features would build on virtio-pci because you get a bunch of things
"for free" - notably enumeration and MSI support. There is assumption
that by the time you add these features to virtio-mmio you end up
re-creating your own less well tested version of virtio-pci. I've not
been terribly convinced by the argument that the guest implementation of
PCI presents a sufficiently large blob of code to make the simpler MMIO
desirable. My attempts to build two virtio kernels (PCI/MMIO) with
otherwise the same devices wasn't terribly conclusive either way.

That said virtio-mmio still has life in it because the cloudy slimmed
down guests moved to using it because the enumeration of PCI is a road
block to their fast boot up requirements. I'm sure they would also
appreciate a MSI implementation to reduce the overhead that handling
notifications currently has on trap-and-emulate.

AIUI for Xen the other downside to PCI is you would have to emulate it
in the hypervisor which would be additional code at the most privileged

> 2. virtio backend is able to access all guest memory, some kind of protection
>    is needed: 'virtio-iommu in Xen' vs 'pre-shared-memory & memcpys in
>    guest'

This is also an area of interest for Project Stratos and something we
would like to be solved generally for all hypervisors. There is a good
write up of some approaches that Jean Phillipe did on the stratos
mailing list:

  From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
  Subject: Limited memory sharing investigation
  Message-ID: <20201002134336.GA2196245@myrica>

I suspect there is a good argument for the simplicity of a combined
virt queue but it is unlikely to be very performance orientated.

> 3. interface between toolstack and 'out-of-qemu' virtio backend, avoid using
>    Xenstore in virtio backend if possible.

I wonder how much work it would be for a rust expert to make:


handle an IOREQ signalling pathway instead of the vhost-user/eventfd
pathway? That would give a good indication on how "hypervisor blind"
these daemons could be made.

> Please note, build-test passed for the following modes:
> 1. x86: CONFIG_HVM=y / CONFIG_IOREQ_SERVER=y (default)
> 2. x86: #CONFIG_HVM is not set / #CONFIG_IOREQ_SERVER is not set

Forgive my relative newness to Xen, how do I convince the hypervisor to
build with this on? I've tried variants of:

  make -j9 CROSS_COMPILE=aarch64-linux-gnu- XEN_TARGET_ARCH=arm64 menuconfig 

with no joy...

> 4. Arm64: CONFIG_HVM=y / #CONFIG_IOREQ_SERVER is not set  (default)
> 6. Arm32: CONFIG_HVM=y / #CONFIG_IOREQ_SERVER is not set  (default)
> ***
> Any feedback/help would be highly appreciated.

Alex Bennée



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