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

From: Ian Jackson <ian.jackson@citrix.com>
To: Paul Durrant <pdurrant@amazon.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	Jason Andryuk <jandryuk@gmail.com>,
	George Dunlap <George.Dunlap@citrix.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v4 5/7] libxl: allow creation of domains with a specified or random domid
Date: Thu, 30 Jan 2020 17:25:55 +0000	[thread overview]
Message-ID: <24115.4515.335032.366833@mariner.uk.xensource.com> (raw)
In-Reply-To: <20200122144446.919-6-pdurrant@amazon.com>

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...

Thanks,
Ian.

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