Re: [Xen-devel] [PATCH v4 5/7] libxl: allow creation of domains with a specified or random domid

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

> -----Original Message-----
> From: Ian Jackson <ian.jackson@xxxxxxxxxx>
> Sent: 30 January 2020 17:26
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu <wl@xxxxxxx>; Anthony Perard
> <anthony.perard@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>;
> George Dunlap <George.Dunlap@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>;
> Julien Grall <julien@xxxxxxx>; Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
> Jason Andryuk <jandryuk@xxxxxxxxx>
> Subject: Re: [PATCH v4 5/7] libxl: allow creation of domains with a
> specified or random domid
> 
> Paul Durrant writes ("[PATCH v4 5/7] libxl: allow creation of domains with
> a specified or random domid"):
> > This patch adds a 'domid' field to libxl_domain_create_info and then
> > modifies libxl__domain_make() to have Xen use that value if it is valid.
> > 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 choose a random domid value
> > for Xen to use.
> >
> > If Xen determines that a domid specified to or chosen by
> > libxl__domain_make() co-incides with an existing domain then the create
> > operation will fail. In this case, if RANDOM_DOMID was specified to
> > libxl__domain_make() then a new random value will be chosen and the
> create
> > operation will be re-tried, otherwise libxl__domain_make() will fail.
> >
> > After Xen has successfully created a new domain, libxl__domain_make()
> will
> > check whether its domid matches any recently used domid values. If it
> does
> > then the domain will be destroyed. If the domid used in creation was
> > specified to libxl__domain_make() then it will fail at this point,
> > otherwise the create operation will be re-tried with either a new random
> > or Xen-selected domid value.
> ...
> > -        ret = xc_domain_create(ctx->xch, domid, &create);
> > +        if (libxl_domid_valid_guest(info->domid))
> > +            *domid = info->domid;
> > +
> > +    again:
> > +        for (;;) {
> > +            if (info->domid == RANDOM_DOMID) {
> > +                uint16_t v;
> > +
> > +                ret = libxl__random_bytes(gc, (void *)&v, sizeof(v));
> > +                if (ret < 0)
> > +                    break;
> > +
> > +                v &= DOMID_MASK;
> > +                if (!libxl_domid_valid_guest(v))
> > +                    continue;
> > +
> > +                *domid = v;
> > +            }
> > +
> > +            ret = xc_domain_create(ctx->xch, domid, &create);
> > +            if (ret == 0 || errno != EEXIST || info->domid !=
> RANDOM_DOMID)
> > +                break;
> > +        }
> > +
> >          if (ret < 0) {
> >              LOGED(ERROR, *domid, "domain creation fail");
> > +            *domid = INVALID_DOMID;
> > +            rc = ERROR_FAIL;
> > +            goto out;
> > +        }
> > +
> > +        if (libxl__is_domid_recent(gc, *domid)) {
> > +            if (*domid == info->domid) /* domid was specified */
> > +                LOGED(ERROR, *domid, "domain id recently used");
> > +
> > +            ret = xc_domain_destroy(ctx->xch, *domid);
> > +            if (!ret) {
> > +                *domid = INVALID_DOMID;
> > +
> > +                /* If the domid was not specified then have another go
> */
> > +                if (!libxl_domid_valid_guest(info->domid))
> > +                    goto again;
> > +            }
> 
> You have written this as two nested loops, one of which is implemented
> as a goto, but actually logically this is surely only one loop ?
> Please could you reorganise this and then I'll read it again...
> 

Ok, I'll try to squash it down to one. It should be do-able.

  Paul

> Thanks,
> Ian.

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