Re: [PATCH V3 00/23] IOREQ feature (+ virtio-mmio) on Arm

[Oleksandr <olekstysh@gmail.com>]

On 30.11.20 18:21, Alex Bennée wrote:

Hi Alex

[added missed subject title]

> Oleksandr Tyshchenko <olekstysh@gmail.com> writes:
>
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>>
>> 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
> level.
Thank you for putting things together here and valuable input. As for 
me, the "virtio-mmio & MSI solution" as a performance improvement sounds 
indeed
interesting. Flipping through the virtio-mmio links I found a discussion 
regarding that [1].
I think this needs an additional investigation and experiments, however 
I am not sure there is an existing infrastructure in Xen on Arm to do so.
Once we make some progress with the IOREQ series I would be able to 
focus on enhancements which we would consider worthwhile.


>
>> 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@linaro.org>
>    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.

I will look at it.


>> 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:
>
>    https://github.com/slp/vhost-user-blk
>
> 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.
>
> <snip>
>> 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
>> 3. Arm64: CONFIG_HVM=y / CONFIG_IOREQ_SERVER=y
> 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 XEN_EXPERT=y [CONFIG_|XEN_|_]IOREQ_SERVER=y
CONFIG_IOREQ_SERVER is not protected by CONFIG_XEN_EXPERT. I mentioned 
how to enable CONFIG_IOREQ_SERVER on Arm (since it is disabled by 
default within this series) when
describing how test this series to Masami, but forgot to add here. Could 
you apply the one-line patch [2] and rebuild. Sorry for inconvenience.


[1] https://lwn.net/Articles/812055/
[2] 
https://github.com/otyshchenko1/xen/commit/b371bc9a3c954595bfce01bad244260364bbcd48

-- 
Regards,

Oleksandr Tyshchenko