Re: [PATCH v4 8/9] tools: add example application to initialize dom0less PV drivers

[Juergen Gross <jgross@xxxxxxxx>]

On 01.04.22 12:21, Julien Grall wrote:
> Hi,
>
> I have posted some comments in v3 after you sent this version. Please have a 
> look.
>
> On 01/04/2022 01:38, Stefano Stabellini wrote:
> > +static int init_domain(struct xs_handle *xsh, libxl_dominfo *info)
> > +{
> > +    struct xc_interface_core *xch;
> > +    libxl_uuid uuid;
> > +    uint64_t xenstore_evtchn, xenstore_pfn;
> > +    int rc;
> > +
> > +    printf("Init dom0less domain: %u\n", info->domid);
> > +    xch = xc_interface_open(0, 0, 0);
> > +
> > +    rc = xc_hvm_param_get(xch, info->domid, HVM_PARAM_STORE_EVTCHN,
> > +                          &xenstore_evtchn);
> > +    if (rc != 0) {
> > +        printf("Failed to get HVM_PARAM_STORE_EVTCHN\n");
> > +        return 1;
> > +    }
> > +
> > +    /* Alloc xenstore page */
> > +    if (alloc_xs_page(xch, info, &xenstore_pfn) != 0) {
> > +        printf("Error on alloc magic pages\n");
> > +        return 1;
> > +    }
> > +
> > +    rc = xc_dom_gnttab_seed(xch, info->domid, true,
> > +                            (xen_pfn_t)-1, xenstore_pfn, 0, 0);
> > +    if (rc)
> > +        err(1, "xc_dom_gnttab_seed");
> > +
> > +    libxl_uuid_generate(&uuid);
> > +    xc_domain_sethandle(xch, info->domid, libxl_uuid_bytearray(&uuid));
> > +
> > +    rc = gen_stub_json_config(info->domid, &uuid);
> > +    if (rc)
> > +        err(1, "gen_stub_json_config");
> > +
> > +    /* Now everything is ready: set HVM_PARAM_STORE_PFN */
> > +    rc = xc_hvm_param_set(xch, info->domid, HVM_PARAM_STORE_PFN,
> > +                          xenstore_pfn);
> On patch #1, you told me you didn't want to allocate the page in Xen because it 
> wouldn't be initialized by Xenstored. But this is what we are doing here.
Xenstore (at least the C variant) is only using the fixed grant ref
GNTTAB_RESERVED_XENSTORE, so it doesn't need the page to be advertised
to the guest. And the mapping is done only when the domain is being
introduced to Xenstore.

> This would be a problem if Linux is still booting and hasn't yet call 
> xenbus_probe_initcall().
> I understand we need to have the page setup before raising the event channel. I 
> don't think we can allow Xenstored to set the HVM_PARAM (it may run in a domain 
> with less privilege). So I think we may need to create a separate command to 
> kick the client (not great).
> Juergen, any thoughts?
I think it should work like that:

- setup the grant via xc_dom_gnttab_seed()
- introduce the domain to Xenstore
- call xc_hvm_param_set()

When the guest is receiving the event, it should wait for the xenstore
page to appear.


Juergen

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