Re: [Xen-devel] [PATCH v4 12/16] libxl: use vchan for QMP access with Linux stubdomain

[Jason Andryuk <jandryuk@gmail.com>]

On Tue, Jan 21, 2020 at 6:46 PM Marek Marczykowski-Górecki
<marmarek@invisiblethingslab.com> wrote:

<snip>

> > > +static void spawn_qmp_proxy(libxl__egc *egc,
> > > +                            libxl__stub_dm_spawn_state *sdss)
> > > +{
> > > +    STATE_AO_GC(sdss->qmp_proxy_spawn.ao);
> > > +    const uint32_t guest_domid = sdss->dm.guest_domid;
> > > +    const uint32_t dm_domid = sdss->pvqemu.guest_domid;
> > > +    const char *dom_path = libxl__xs_get_dompath(gc, dm_domid);
> > > +    char **args;
> > > +    int nr = 0;
> > > +    int rc, logfile_w, null;
> > > +
> > > +    if (access(STUBDOM_QMP_PROXY_PATH, X_OK) < 0) {
> > > +        LOGED(ERROR, guest_domid, "qmp proxy %s is not executable", STUBDOM_QMP_PROXY_PATH);
> > > +        rc = ERROR_FAIL;
> > > +        goto out;
> > > +    }
> > > +
> > > +    sdss->qmp_proxy_spawn.what = GCSPRINTF("domain %d device model qmp proxy", guest_domid);
> > > +    sdss->qmp_proxy_spawn.pidpath = GCSPRINTF("%s/image/qmp-proxy-pid", dom_path);
> > > +    sdss->qmp_proxy_spawn.xspath = GCSPRINTF("%s/image/qmp-proxy-state", dom_path);
> >
> > Since this is the vchan-socket-proxy in dom0, should it write to
> > "device-model/%u/qmp-proxy-state" underneath dom0?
>
> Yes, that would be more consistent. But pid should stay where it is
> (it's also where dom0 qemu pid is being written).

Hmm, that split between pids and device-model info is a little weird.
But it is specified in docs misc/xenstore-paths.pandoc

> > > +
> > > +    sdss->qmp_proxy_spawn.timeout_ms = LIBXL_DEVICE_MODEL_START_TIMEOUT * 1000;
> > > +    sdss->qmp_proxy_spawn.midproc_cb = libxl__spawn_record_pid;
> > > +    sdss->qmp_proxy_spawn.confirm_cb = qmp_proxy_confirm;
> > > +    sdss->qmp_proxy_spawn.failure_cb = qmp_proxy_startup_failed;
> > > +    sdss->qmp_proxy_spawn.detached_cb = qmp_proxy_detached;
> > > +
> > > +    const int arraysize = 6;
> > > +    GCNEW_ARRAY(args, arraysize);
> > > +    args[nr++] = STUBDOM_QMP_PROXY_PATH;
> > > +    args[nr++] = GCSPRINTF("--state-path=%s", sdss->qmp_proxy_spawn.xspath);
> > > +    args[nr++] = GCSPRINTF("%u", dm_domid);
> > > +    args[nr++] = GCSPRINTF("%s/device-model/%u/qmp-vchan", dom_path, guest_domid);
> >
> > Thinking of OpenXT"s qmp-helper, this path isn't useful.  But it is
> > for vchan-socket-proxy, so qmp-helper could just change to ignore it.
>
> For vchan we could use also a port number (and then it will encode it
> into a xenstore path). This is in fact how we use libvchan in Qubes. I
> opted for explicit path only because of lack of idea for some meaningful
> port number ;) But I'm open for suggestions.
> I guess that would be useful for Argo version then.

The argo version hard codes the port number, so it's not a command
line argument.  The port number would need to get passed to the
stubdom or it would need to be standardized.

I think the arguments for vchan-socket-proxy make sense.  Since it's
the one that's submitted upstream, it makes sense to use them.

Put another way, do we want this to support alternate implementations
for a qmp proxy?  Should the arguments be generic for that case?
Since the existing arguments have enough information for either proxy,
I think it's fine to leave it as is.  While you could have a wrapper
generate all the above from just the domid & stub_domid, that's kinda
hacky.

Thanks,
Jason

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