Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of domains with a specified or random domid

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 13 January 2020 22:24
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>; jandryuk@xxxxxxxxx
> Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>; xen-devel <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>; Ian Jackson <ian.jackson@xxxxxxxxxxxxx>; Wei
> Liu <wl@xxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of domains
> with a specified or random domid
> 
> Hi Paul,
> 
> On 13/01/2020 16:54, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: jandryuk@xxxxxxxxx <jandryuk@xxxxxxxxx>
> >> Sent: 13 January 2020 16:16
> >> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> >> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Anthony PERARD
> >> <anthony.perard@xxxxxxxxxx>; Ian Jackson <ian.jackson@xxxxxxxxxxxxx>;
> Wei
> >> Liu <wl@xxxxxxx>
> >> Subject: Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of
> domains
> >> with a specified or random domid
> >>
> >> On Thu, Jan 9, 2020 at 6:50 AM Paul Durrant <pdurrant@xxxxxxxxxx>
> wrote:
> >>>
> >>> This patch adds a 'domid' field to libxl_domain_create_info and then
> >>> modifies do_domain_create() to use that value if it is valid. Any
> valid
> >>> domid will be checked against the retired domid list before being
> passed
> >>> to libxl__domain_make().
> >>> If the domid value is invalid then Xen will choose the domid, as
> before,
> >>> unless the value is the new special RANDOM_DOMID value added to the
> API.
> >>> This value instructs libxl__domain_make() to select a random domid
> >> value,
> >>> check it for validity, verify it does not match a retired domain, and
> >> then
> >>> pass it to Xen's XEN_DOMCTL_createdomain operation. If Xen determines
> >> that
> >>> it co-incides with an existing domain, a new random value will be
> >>> selected and the operation will be re-tried.
> >>>
> >>> NOTE: libxl__logv() is also modified to only log valid domid values in
> >>>        messages rather than any domid, valid or otherwise, that is not
> >>>        INVALID_DOMID.
> >>>
> >>> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> >>> ---
> >>> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> >>> Cc: Wei Liu <wl@xxxxxxx>
> >>> Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> >>>
> >>> v2:
> >>>   - Re-worked to use a value from libxl_domain_create_info
> >>> ---
> >>>   tools/libxl/libxl.h          |  9 +++++++++
> >>>   tools/libxl/libxl_create.c   | 32 +++++++++++++++++++++++++++++++-
> >>>   tools/libxl/libxl_internal.c |  2 +-
> >>>   tools/libxl/libxl_types.idl  |  1 +
> >>>   4 files changed, 42 insertions(+), 2 deletions(-)
> >>>
> >>
> >> <snip>
> >>
> >>> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> >>> index 1835a5502c..ee76dee364 100644
> >>> --- a/tools/libxl/libxl_create.c
> >>> +++ b/tools/libxl/libxl_create.c
> >>> @@ -600,9 +600,39 @@ int libxl__domain_make(libxl__gc *gc,
> >> libxl_domain_config *d_config,
> >>>               goto out;
> >>>           }
> >>>
> >>> -        ret = xc_domain_create(ctx->xch, domid, &create);
> >>> +        if (libxl_domid_valid_guest(info->domid)) {
> >>> +            *domid = info->domid;
> >>> +
> >>> +            if (libxl__is_retired_domid(gc, *domid)) {
> >>> +                LOGED(ERROR, *domid, "domain id is retired");
> >>> +                rc = ERROR_FAIL;
> >>> +                goto out;
> >>> +            }
> >>> +        } else if (info->domid == RANDOM_DOMID) {
> >>> +            *domid = 0; /* Zero-out initial value */
> >>> +        }
> >>> +
> >>> +        for (;;) {
> >>> +            if (info->domid == RANDOM_DOMID) {
> >>> +                /* Randomize lower order bytes */
> >>> +                ret = libxl__random_bytes(gc, (void *)domid,
> >>> +                                          sizeof(uint16_t));
> >>
> >> Casting to void * assumes little endian.
> >
> > I think that's a fairly safe assumption as far as Xen goes...
> 
> Not really, there are technically nothing (other than bug fixes)
> preventing us to use a big endian guest on Xen on Arm.
>

Ok.
 
> I actually did play with big endian on Xen in the past and managed to
> get a guest running. The main annoying part is Linux as it is assuming
> to use the same endian as the hypervisor. But other OS may not have this
> issues...
> 
> The hypervisor itself is likely going to stay little endian, so does the
> interface. For the tools, we should aim to not introduce more assumption
> that the software will be little endian.
> 

Fair enough. If there's a realistic possibility of running a BE tools domain 
then I'll code accordingly.

  Paul

> Cheers,
> 
> --
> Julien Grall
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