Re: [openxt-dev] VirtIO-Argo initial development proposal

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Dec 30, 2020 at 11:30:03AM +0000, Julien Grall wrote:
> Hi Roger,
> 
> On 29/12/2020 09:17, Roger Pau Monné wrote:
> > On Wed, Dec 23, 2020 at 04:32:01PM -0500, Rich Persaud wrote:
> > > On Dec 17, 2020, at 07:13, Jean-Philippe Ouellet <jpo@xxxxxx> wrote:
> > > > On Wed, Dec 16, 2020 at 2:37 PM Christopher Clark
> > > > <christopher.w.clark@xxxxxxxxx> wrote:
> > > > > Hi all,
> > > > > 
> > > > > I have written a page for the OpenXT wiki describing a proposal for
> > > > > initial development towards the VirtIO-Argo transport driver, and the
> > > > > related system components to support it, destined for OpenXT and
> > > > > upstream projects:
> > > > > 
> > > > > https://openxt.atlassian.net/wiki/spaces/~cclark/pages/1696169985/VirtIO-Argo+Development+Phase+1
> > 
> > Thanks for the detailed document, I've taken a look and there's indeed
> > a lot of work to do listed there :). I have some suggestion and
> > questions.
> > 
> > Overall I think it would be easier for VirtIO to take a new transport
> > if it's not tied to a specific hypervisor. The way Argo is implemented
> > right now is using hypercalls, which is a mechanism specific to Xen.
> The concept of hypervisor call is not Xen specific. Any other hypervisor can
> easily implement them. At least this is the case on Arm because we have an
> instruction 'hvc' that acts the same way as a syscall but for the
> hypervisor.
> 
> What we would need to do is reserving a range for Argos. It should be
> possible to do it on Arm thanks to the SMCCC (see [1]).
> 
> I am not sure whether you have something similar on x86.

On x86 Intel has vmcall and AMD vmmcall, but those are only available
to HVM guests.

> > IMO it might be easier to start by having an Argo interface using
> > MSRs, that all hypervisors can implement, and then base the VirtIO
> > implementation on top of that interface.
> My concern is the interface would need to be arch-specific. Would you mind
> to explain what's the problem to implement something based on vmcall?

More of a recommendation than a concern, but I think it would be more
attractive for upstream if we could provide an interface to Argo (or
hypervisor mediated data exchange) that's no too tied into Xen
specifics. Using a defined vmcall/vmmcall interface (and leaving PV out
of the picture?) could be one option.

My suggestion for using MSRs was because I think every x86 hypervisor
must have the logic to trap and handle some of those, and also every
OS has the helpers to read/write MSRs, and those instructions are not
vendor specific.

Roger.