From: Luca Miccio <lucmiccio@gmail.com>

Add an example application that can be run in dom0 to complete the
dom0less domains initialization so that they can get access to xenstore
and use PV drivers.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
CC: Wei Liu <wl@xen.org>
CC: Anthony PERARD <anthony.perard@citrix.com>
CC: Juergen Gross <jgross@suse.com>
---
Changes in v2:
- do not set HVM_PARAM_STORE_EVTCHN twice
- rename restore_xenstore to create_xenstore
- increase maxmem
---
 tools/helpers/Makefile        |  13 ++
 tools/helpers/init-dom0less.c | 266 ++++++++++++++++++++++++++++++++++
 2 files changed, 279 insertions(+)
 create mode 100644 tools/helpers/init-dom0less.c

diff --git a/tools/helpers/Makefile b/tools/helpers/Makefile
index 7f6c422440..8e42997052 100644
--- a/tools/helpers/Makefile
+++ b/tools/helpers/Makefile
@@ -10,6 +10,9 @@ ifeq ($(CONFIG_Linux),y)
 ifeq ($(CONFIG_X86),y)
 PROGS += init-xenstore-domain
 endif
+ifeq ($(CONFIG_ARM),y)
+PROGS += init-dom0less
+endif
 endif
 
 XEN_INIT_DOM0_OBJS = xen-init-dom0.o init-dom-json.o
@@ -26,6 +29,13 @@ $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += $(CFLAGS_libxenstore)
 $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += $(CFLAGS_libxenlight)
 $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += -include $(XEN_ROOT)/tools/config.h
 
+INIT_DOM0LESS_OBJS = init-dom0less.o init-dom-json.o
+$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxentoollog)
+$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenstore)
+$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenlight)
+$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenctrl)
+$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenevtchn)
+
 .PHONY: all
 all: $(PROGS)
 
@@ -35,6 +45,9 @@ xen-init-dom0: $(XEN_INIT_DOM0_OBJS)
 init-xenstore-domain: $(INIT_XENSTORE_DOMAIN_OBJS)
 	$(CC) $(LDFLAGS) -o $@ $(INIT_XENSTORE_DOMAIN_OBJS) $(LDLIBS_libxentoollog) $(LDLIBS_libxenstore) $(LDLIBS_libxenctrl) $(LDLIBS_libxenguest) $(LDLIBS_libxenlight) $(APPEND_LDFLAGS)
 
+init-dom0less: $(INIT_DOM0LESS_OBJS)
+	$(CC) $(LDFLAGS) -o $@ $(INIT_DOM0LESS_OBJS) $(LDLIBS_libxenctrl) $(LDLIBS_libxenevtchn) $(LDLIBS_libxentoollog) $(LDLIBS_libxenstore) $(LDLIBS_libxenlight) $(LDLIBS_libxenguest)  $(APPEND_LDFLAGS)
+
 .PHONY: install
 install: all
 	$(INSTALL_DIR) $(DESTDIR)$(LIBEXEC_BIN)
diff --git a/tools/helpers/init-dom0less.c b/tools/helpers/init-dom0less.c
new file mode 100644
index 0000000000..0434ca064f
--- /dev/null
+++ b/tools/helpers/init-dom0less.c
@@ -0,0 +1,266 @@
+#include <stdbool.h>
+#include <syslog.h>
+#include <stdio.h>
+#include <err.h>
+#include <stdlib.h>
+#include <xenstore.h>
+#include <xenctrl.h>
+#include <xenguest.h>
+#include <libxl.h>
+#include <xenevtchn.h>
+
+#include "init-dom-json.h"
+
+#define NR_MAGIC_PAGES 4
+#define CONSOLE_PFN_OFFSET 0
+#define XENSTORE_PFN_OFFSET 1
+#define STR_MAX_LENGTH 64
+
+static int alloc_magic_pages(libxl_dominfo *info, struct xc_dom_image *dom)
+{
+    int rc, i;
+    const xen_pfn_t base = GUEST_MAGIC_BASE >> XC_PAGE_SHIFT;
+    xen_pfn_t p2m[NR_MAGIC_PAGES];
+
+    rc = xc_domain_setmaxmem(dom->xch, dom->guest_domid,
+                             info->max_memkb + NR_MAGIC_PAGES * 4);
+    if (rc < 0)
+        return rc;
+
+    for (i = 0; i < NR_MAGIC_PAGES; i++)
+        p2m[i] = base + i;
+
+    rc = xc_domain_populate_physmap_exact(dom->xch, dom->guest_domid,
+                                          NR_MAGIC_PAGES, 0, 0, p2m);
+    if (rc < 0)
+        return rc;
+
+    dom->xenstore_pfn = base + XENSTORE_PFN_OFFSET;
+
+    xc_clear_domain_page(dom->xch, dom->guest_domid, dom->xenstore_pfn);
+
+    xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_STORE_PFN,
+                     dom->xenstore_pfn);
+    return 0;
+}
+
+static void do_xs_write(struct xs_handle *xsh, xs_transaction_t t,
+                        char *path, char *val)
+{
+    if (!xs_write(xsh, t, path, val, strlen(val)))
+        fprintf(stderr, "writing %s to xenstore failed.\n", path);
+}
+
+static void do_xs_write_dom(struct xs_handle *xsh, xs_transaction_t t,
+                            domid_t domid, char *path, char *val)
+{
+    char full_path[STR_MAX_LENGTH];
+
+    snprintf(full_path, STR_MAX_LENGTH,
+             "/local/domain/%d/%s", domid, path);
+    do_xs_write(xsh, t, full_path, val);
+}
+
+static void do_xs_write_libxl(struct xs_handle *xsh, xs_transaction_t t,
+                              domid_t domid, char *path, char *val)
+{
+    char full_path[STR_MAX_LENGTH];
+
+    snprintf(full_path, STR_MAX_LENGTH,
+             "/libxl/%d/%s", domid, path);
+    do_xs_write(xsh, t, full_path, val);
+}
+
+static void do_xs_write_vm(struct xs_handle *xsh, xs_transaction_t t,
+                           libxl_uuid uuid, char *path, char *val)
+{
+    char full_path[STR_MAX_LENGTH];
+
+    snprintf(full_path, STR_MAX_LENGTH,
+             "/vm/" LIBXL_UUID_FMT "/%s", LIBXL_UUID_BYTES(uuid), path);
+    do_xs_write(xsh, t, full_path, val);
+}
+
+static int restore_xenstore(struct xs_handle *xsh,
+                            libxl_dominfo *info, libxl_uuid uuid,
+                            evtchn_port_t xenstore_port)
+{
+    domid_t domid;
+    int i;
+    char uuid_str[STR_MAX_LENGTH];
+    char dom_name_str[STR_MAX_LENGTH];
+    char vm_val_str[STR_MAX_LENGTH];
+    char id_str[STR_MAX_LENGTH];
+    char max_memkb_str[STR_MAX_LENGTH];
+    char cpu_str[STR_MAX_LENGTH];
+    char xenstore_port_str[STR_MAX_LENGTH];
+    char ring_ref_str[STR_MAX_LENGTH];
+    xs_transaction_t t;
+
+    domid = info->domid;
+    snprintf(id_str, STR_MAX_LENGTH, "%d", domid);
+    snprintf(dom_name_str, STR_MAX_LENGTH, "dom0less-%d", domid);
+    snprintf(uuid_str, STR_MAX_LENGTH, LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
+    snprintf(vm_val_str, STR_MAX_LENGTH,
+             "vm/" LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
+    snprintf(max_memkb_str, STR_MAX_LENGTH, "%lu", info->max_memkb);
+    snprintf(ring_ref_str, STR_MAX_LENGTH, "%lld",
+             (GUEST_MAGIC_BASE >> XC_PAGE_SHIFT) + XENSTORE_PFN_OFFSET);
+    snprintf(xenstore_port_str, STR_MAX_LENGTH, "%d", xenstore_port);
+
+retry_transaction:
+    t = xs_transaction_start(xsh);
+    if (t == XBT_NULL)
+        return errno;
+
+    /* /vm */
+    do_xs_write_vm(xsh, t, uuid, "name", dom_name_str);
+    do_xs_write_vm(xsh, t, uuid, "uuid", uuid_str);
+    do_xs_write_vm(xsh, t, uuid, "start_time", "0");
+
+    /* /domain */
+    do_xs_write_dom(xsh, t, domid, "vm", vm_val_str);
+    do_xs_write_dom(xsh, t, domid, "name", dom_name_str);
+    do_xs_write_dom(xsh, t, domid, "cpu", "");
+    for (i = 0; i < info->vcpu_max_id; i++) {
+        snprintf(cpu_str, STR_MAX_LENGTH, "cpu/%d/availability/", i);
+        do_xs_write_dom(xsh, t, domid, cpu_str,
+                        (info->cpupool & (1 << i)) ? "online" : "offline");
+    }
+    do_xs_write_dom(xsh, t, domid, "cpu/0", "");
+    do_xs_write_dom(xsh, t, domid, "cpu/availability", "online");
+
+    do_xs_write_dom(xsh, t, domid, "memory", "");
+    do_xs_write_dom(xsh, t, domid, "memory/static-max", max_memkb_str);
+    do_xs_write_dom(xsh, t, domid, "memory/videoram", "-1");
+
+    do_xs_write_dom(xsh, t, domid, "device", "");
+    do_xs_write_dom(xsh, t, domid, "device/suspend", "");
+    do_xs_write_dom(xsh, t, domid, "device/suspend/event-channel", "");
+
+    do_xs_write_dom(xsh, t, domid, "control", "");
+    do_xs_write_dom(xsh, t, domid, "control/shutdown", "");
+    do_xs_write_dom(xsh, t, domid, "control/feature-poweroff", "1");
+    do_xs_write_dom(xsh, t, domid, "control/feature-reboot", "1");
+    do_xs_write_dom(xsh, t, domid, "control/feature-suspend", "");
+    do_xs_write_dom(xsh, t, domid, "control/sysrq", "");
+    do_xs_write_dom(xsh, t, domid, "control/platform-feature-multiprocessor-suspend", "1");
+    do_xs_write_dom(xsh, t, domid, "control", "platform-feature-xs_reset_watches");
+
+    do_xs_write_dom(xsh, t, domid, "domid", id_str);
+    do_xs_write_dom(xsh, t, domid, "data", "");
+    do_xs_write_dom(xsh, t, domid, "drivers", "");
+    do_xs_write_dom(xsh, t, domid, "feature", "");
+    do_xs_write_dom(xsh, t, domid, "attr", "");
+
+    do_xs_write_dom(xsh, t, domid, "store/port", xenstore_port_str);
+    do_xs_write_dom(xsh, t, domid, "store/ring-ref", ring_ref_str);
+
+    do_xs_write_libxl(xsh, t, domid, "type", "pvh");
+    do_xs_write_libxl(xsh, t, domid, "dm-version", "qemu_xen");
+
+    if (!xs_transaction_end(xsh, t, false))
+        if (errno == EAGAIN)
+            goto retry_transaction;
+
+    return 0;
+}
+
+static int init_domain(struct xs_handle *xsh, libxl_dominfo *info)
+{
+    struct xc_dom_image dom;
+    libxl_uuid uuid;
+    uint64_t v;
+    int rc;
+
+    printf("#### Init dom0less domain: %d ####\n", info->domid);
+    dom.guest_domid = info->domid;
+    dom.xenstore_domid = 0;
+    dom.xch = xc_interface_open(0, 0, 0);
+
+    rc = xc_hvm_param_get(dom.xch, info->domid, HVM_PARAM_STORE_EVTCHN, &v);
+    if (rc != 0) {
+        printf("Failed to get HVM_PARAM_STORE_EVTCHN\n");
+        return 1;
+    }
+    dom.xenstore_evtchn = v;
+
+    /* Console won't be initialized but set its data for completeness */
+    dom.console_domid = 0;
+
+    /* Alloc magic pages */
+    printf("Allocating magic pages\n");
+    if (alloc_magic_pages(info, &dom) != 0) {
+        printf("Error on alloc magic pages\n");
+        return 1;
+    }
+
+    printf("Setup Grant Tables\n");
+    xc_dom_gnttab_init(&dom);
+
+    printf("Setup UUID\n");
+    libxl_uuid_generate(&uuid);
+    xc_domain_sethandle(dom.xch, info->domid, libxl_uuid_bytearray(&uuid));
+
+    printf("Creating JSON\n");
+    rc = gen_stub_json_config(info->domid, &uuid);
+    if (rc)
+        err(1, "gen_stub_json_config");
+
+    printf("Restoring Xenstore values\n");
+    restore_xenstore(xsh, info, uuid, dom.xenstore_evtchn);
+
+    printf("Introducing domain\n");
+    xs_introduce_domain(xsh, info->domid,
+            (GUEST_MAGIC_BASE >> XC_PAGE_SHIFT) + XENSTORE_PFN_OFFSET,
+            dom.xenstore_evtchn);
+    return 0;
+}
+
+/* Check if domain has been configured in XS */
+static bool domain_exists(struct xs_handle *xsh, int domid)
+{
+    return xs_is_domain_introduced(xsh, domid);
+}
+
+int main(int argc, char **argv)
+{
+    libxl_dominfo *info;
+    libxl_ctx *ctx;
+    int nb_vm, i;
+    struct xs_handle *xsh;
+
+    xsh = xs_daemon_open();
+    if (xsh == NULL) {
+        fprintf(stderr, "Could not contact XenStore");
+        exit(1);
+    }
+
+    if (libxl_ctx_alloc(&ctx, LIBXL_VERSION, 0, NULL)) {
+        fprintf(stderr, "cannot init xl context\n");
+        exit(1);
+    }
+
+    info = libxl_list_domain(ctx, &nb_vm);
+    if (!info) {
+        fprintf(stderr, "libxl_list_vm failed.\n");
+        exit(EXIT_FAILURE);
+    }
+
+    for (i = 0; i < nb_vm; i++) {
+        domid_t domid = info[i].domid;
+
+        /* Don't need to check for Dom0 */
+        if (!domid)
+            continue;
+
+        printf("Checking domid: %u\n", domid);
+        if (!domain_exists(xsh, domid))
+            init_domain(xsh, &info[i]);
+        else
+            printf("Domain %d has already been initialized\n", domid);
+    }
+    libxl_dominfo_list_free(info, nb_vm);
+    xs_close(xsh);
+    return 0;
+}
-- 
2.25.1

Hi Stefano,

> On 13 Jan 2022, at 00:58, Stefano Stabellini <sstabellini@kernel.org> wrote:
> 
> From: Luca Miccio <lucmiccio@gmail.com>
> 
> Add an example application that can be run in dom0 to complete the
> dom0less domains initialization so that they can get access to xenstore
> and use PV drivers.
> 
> Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> CC: Wei Liu <wl@xen.org>
> CC: Anthony PERARD <anthony.perard@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> ---
> Changes in v2:
> - do not set HVM_PARAM_STORE_EVTCHN twice
> - rename restore_xenstore to create_xenstore
> - increase maxmem
> ---
> tools/helpers/Makefile        |  13 ++
> tools/helpers/init-dom0less.c | 266 ++++++++++++++++++++++++++++++++++
> 2 files changed, 279 insertions(+)
> create mode 100644 tools/helpers/init-dom0less.c
> 
> diff --git a/tools/helpers/Makefile b/tools/helpers/Makefile
> index 7f6c422440..8e42997052 100644
> --- a/tools/helpers/Makefile
> +++ b/tools/helpers/Makefile
> @@ -10,6 +10,9 @@ ifeq ($(CONFIG_Linux),y)
> ifeq ($(CONFIG_X86),y)
> PROGS += init-xenstore-domain
> endif
> +ifeq ($(CONFIG_ARM),y)
> +PROGS += init-dom0less
> +endif
> endif
> 
> XEN_INIT_DOM0_OBJS = xen-init-dom0.o init-dom-json.o
> @@ -26,6 +29,13 @@ $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += $(CFLAGS_libxenstore)
> $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += $(CFLAGS_libxenlight)
> $(INIT_XENSTORE_DOMAIN_OBJS): CFLAGS += -include $(XEN_ROOT)/tools/config.h
> 
> +INIT_DOM0LESS_OBJS = init-dom0less.o init-dom-json.o
> +$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxentoollog)
> +$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenstore)
> +$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenlight)
> +$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenctrl)
> +$(INIT_DOM0LESS_OBJS): CFLAGS += $(CFLAGS_libxenevtchn)
> +
> .PHONY: all
> all: $(PROGS)
> 
> @@ -35,6 +45,9 @@ xen-init-dom0: $(XEN_INIT_DOM0_OBJS)
> init-xenstore-domain: $(INIT_XENSTORE_DOMAIN_OBJS)
> 	$(CC) $(LDFLAGS) -o $@ $(INIT_XENSTORE_DOMAIN_OBJS) $(LDLIBS_libxentoollog) $(LDLIBS_libxenstore) $(LDLIBS_libxenctrl) $(LDLIBS_libxenguest) $(LDLIBS_libxenlight) $(APPEND_LDFLAGS)
> 
> +init-dom0less: $(INIT_DOM0LESS_OBJS)
> +	$(CC) $(LDFLAGS) -o $@ $(INIT_DOM0LESS_OBJS) $(LDLIBS_libxenctrl) $(LDLIBS_libxenevtchn) $(LDLIBS_libxentoollog) $(LDLIBS_libxenstore) $(LDLIBS_libxenlight) $(LDLIBS_libxenguest)  $(APPEND_LDFLAGS)
> +
> .PHONY: install
> install: all
> 	$(INSTALL_DIR) $(DESTDIR)$(LIBEXEC_BIN)
> diff --git a/tools/helpers/init-dom0less.c b/tools/helpers/init-dom0less.c
> new file mode 100644
> index 0000000000..0434ca064f
> --- /dev/null
> +++ b/tools/helpers/init-dom0less.c
> @@ -0,0 +1,266 @@
> +#include <stdbool.h>
> +#include <syslog.h>
> +#include <stdio.h>
> +#include <err.h>
> +#include <stdlib.h>
> +#include <xenstore.h>
> +#include <xenctrl.h>
> +#include <xenguest.h>
> +#include <libxl.h>
> +#include <xenevtchn.h>
> +
> +#include "init-dom-json.h"
> +
> +#define NR_MAGIC_PAGES 4
> +#define CONSOLE_PFN_OFFSET 0
> +#define XENSTORE_PFN_OFFSET 1
> +#define STR_MAX_LENGTH 64
> +
> +static int alloc_magic_pages(libxl_dominfo *info, struct xc_dom_image *dom)
> +{
> +    int rc, i;
> +    const xen_pfn_t base = GUEST_MAGIC_BASE >> XC_PAGE_SHIFT;
> +    xen_pfn_t p2m[NR_MAGIC_PAGES];
> +
> +    rc = xc_domain_setmaxmem(dom->xch, dom->guest_domid,
> +                             info->max_memkb + NR_MAGIC_PAGES * 4);
> +    if (rc < 0)
> +        return rc;
> +
> +    for (i = 0; i < NR_MAGIC_PAGES; i++)
> +        p2m[i] = base + i;
> +
> +    rc = xc_domain_populate_physmap_exact(dom->xch, dom->guest_domid,
> +                                          NR_MAGIC_PAGES, 0, 0, p2m);
> +    if (rc < 0)
> +        return rc;
> +
> +    dom->xenstore_pfn = base + XENSTORE_PFN_OFFSET;
> +
> +    xc_clear_domain_page(dom->xch, dom->guest_domid, dom->xenstore_pfn);
> +
> +    xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_STORE_PFN,
> +                     dom->xenstore_pfn);
> +    return 0;
> +}
> +
> +static void do_xs_write(struct xs_handle *xsh, xs_transaction_t t,
> +                        char *path, char *val)
> +{
> +    if (!xs_write(xsh, t, path, val, strlen(val)))
> +        fprintf(stderr, "writing %s to xenstore failed.\n", path);

Error here is ignored, do we really want that ?

> +}
> +
> +static void do_xs_write_dom(struct xs_handle *xsh, xs_transaction_t t,
> +                            domid_t domid, char *path, char *val)
> +{
> +    char full_path[STR_MAX_LENGTH];
> +
> +    snprintf(full_path, STR_MAX_LENGTH,
> +             "/local/domain/%d/%s", domid, path);
> +    do_xs_write(xsh, t, full_path, val);
> +}
> +
> +static void do_xs_write_libxl(struct xs_handle *xsh, xs_transaction_t t,
> +                              domid_t domid, char *path, char *val)
> +{
> +    char full_path[STR_MAX_LENGTH];
> +
> +    snprintf(full_path, STR_MAX_LENGTH,
> +             "/libxl/%d/%s", domid, path);
> +    do_xs_write(xsh, t, full_path, val);
> +}
> +
> +static void do_xs_write_vm(struct xs_handle *xsh, xs_transaction_t t,
> +                           libxl_uuid uuid, char *path, char *val)
> +{
> +    char full_path[STR_MAX_LENGTH];
> +
> +    snprintf(full_path, STR_MAX_LENGTH,
> +             "/vm/" LIBXL_UUID_FMT "/%s", LIBXL_UUID_BYTES(uuid), path);
> +    do_xs_write(xsh, t, full_path, val);
> +}
> +
> +static int restore_xenstore(struct xs_handle *xsh,
> +                            libxl_dominfo *info, libxl_uuid uuid,
> +                            evtchn_port_t xenstore_port)
> +{
> +    domid_t domid;
> +    int i;
> +    char uuid_str[STR_MAX_LENGTH];
> +    char dom_name_str[STR_MAX_LENGTH];
> +    char vm_val_str[STR_MAX_LENGTH];
> +    char id_str[STR_MAX_LENGTH];
> +    char max_memkb_str[STR_MAX_LENGTH];
> +    char cpu_str[STR_MAX_LENGTH];
> +    char xenstore_port_str[STR_MAX_LENGTH];
> +    char ring_ref_str[STR_MAX_LENGTH];
> +    xs_transaction_t t;
> +
> +    domid = info->domid;
> +    snprintf(id_str, STR_MAX_LENGTH, "%d", domid);
> +    snprintf(dom_name_str, STR_MAX_LENGTH, "dom0less-%d", domid);
> +    snprintf(uuid_str, STR_MAX_LENGTH, LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
> +    snprintf(vm_val_str, STR_MAX_LENGTH,
> +             "vm/" LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
> +    snprintf(max_memkb_str, STR_MAX_LENGTH, "%lu", info->max_memkb);
> +    snprintf(ring_ref_str, STR_MAX_LENGTH, "%lld",
> +             (GUEST_MAGIC_BASE >> XC_PAGE_SHIFT) + XENSTORE_PFN_OFFSET);
> +    snprintf(xenstore_port_str, STR_MAX_LENGTH, "%d", xenstore_port);
> +
> +retry_transaction:
> +    t = xs_transaction_start(xsh);
> +    if (t == XBT_NULL)
> +        return errno;
> +

All the following is very static and for a dummy reviewer (like me) kind of magic.
Could you add a comment on top of this explaining how this list of entries was
created so that someone updating/reviewing this code can make sure it is right ?

> +    /* /vm */
> +    do_xs_write_vm(xsh, t, uuid, "name", dom_name_str);
> +    do_xs_write_vm(xsh, t, uuid, "uuid", uuid_str);
> +    do_xs_write_vm(xsh, t, uuid, "start_time", "0");
> +
> +    /* /domain */
> +    do_xs_write_dom(xsh, t, domid, "vm", vm_val_str);
> +    do_xs_write_dom(xsh, t, domid, "name", dom_name_str);
> +    do_xs_write_dom(xsh, t, domid, "cpu", "");
> +    for (i = 0; i < info->vcpu_max_id; i++) {
> +        snprintf(cpu_str, STR_MAX_LENGTH, "cpu/%d/availability/", i);
> +        do_xs_write_dom(xsh, t, domid, cpu_str,
> +                        (info->cpupool & (1 << i)) ? "online" : "offline");
> +    }
> +    do_xs_write_dom(xsh, t, domid, "cpu/0", "");
> +    do_xs_write_dom(xsh, t, domid, "cpu/availability", "online");
> +
> +    do_xs_write_dom(xsh, t, domid, "memory", "");
> +    do_xs_write_dom(xsh, t, domid, "memory/static-max", max_memkb_str);
> +    do_xs_write_dom(xsh, t, domid, "memory/videoram", "-1");
> +
> +    do_xs_write_dom(xsh, t, domid, "device", "");
> +    do_xs_write_dom(xsh, t, domid, "device/suspend", "");
> +    do_xs_write_dom(xsh, t, domid, "device/suspend/event-channel", "");
> +
> +    do_xs_write_dom(xsh, t, domid, "control", "");
> +    do_xs_write_dom(xsh, t, domid, "control/shutdown", "");
> +    do_xs_write_dom(xsh, t, domid, "control/feature-poweroff", "1");
> +    do_xs_write_dom(xsh, t, domid, "control/feature-reboot", "1");
> +    do_xs_write_dom(xsh, t, domid, "control/feature-suspend", "");
> +    do_xs_write_dom(xsh, t, domid, "control/sysrq", "");
> +    do_xs_write_dom(xsh, t, domid, "control/platform-feature-multiprocessor-suspend", "1");
> +    do_xs_write_dom(xsh, t, domid, "control", "platform-feature-xs_reset_watches");
> +
> +    do_xs_write_dom(xsh, t, domid, "domid", id_str);
> +    do_xs_write_dom(xsh, t, domid, "data", "");
> +    do_xs_write_dom(xsh, t, domid, "drivers", "");
> +    do_xs_write_dom(xsh, t, domid, "feature", "");
> +    do_xs_write_dom(xsh, t, domid, "attr", "");
> +
> +    do_xs_write_dom(xsh, t, domid, "store/port", xenstore_port_str);
> +    do_xs_write_dom(xsh, t, domid, "store/ring-ref", ring_ref_str);
> +
> +    do_xs_write_libxl(xsh, t, domid, "type", "pvh");
> +    do_xs_write_libxl(xsh, t, domid, "dm-version", "qemu_xen");
> +
> +    if (!xs_transaction_end(xsh, t, false))
> +        if (errno == EAGAIN)
> +            goto retry_transaction;
> +
> +    return 0;
> +}
> +
> +static int init_domain(struct xs_handle *xsh, libxl_dominfo *info)
> +{
> +    struct xc_dom_image dom;
> +    libxl_uuid uuid;
> +    uint64_t v;
> +    int rc;
> +
> +    printf("#### Init dom0less domain: %d ####\n", info->domid);
> +    dom.guest_domid = info->domid;
> +    dom.xenstore_domid = 0;
> +    dom.xch = xc_interface_open(0, 0, 0);
> +
> +    rc = xc_hvm_param_get(dom.xch, info->domid, HVM_PARAM_STORE_EVTCHN, &v);
> +    if (rc != 0) {
> +        printf("Failed to get HVM_PARAM_STORE_EVTCHN\n");
> +        return 1;
> +    }
> +    dom.xenstore_evtchn = v;
> +
> +    /* Console won't be initialized but set its data for completeness */
> +    dom.console_domid = 0;
> +
> +    /* Alloc magic pages */
> +    printf("Allocating magic pages\n");
> +    if (alloc_magic_pages(info, &dom) != 0) {
> +        printf("Error on alloc magic pages\n");
> +        return 1;
> +    }
> +
> +    printf("Setup Grant Tables\n");
> +    xc_dom_gnttab_init(&dom);
> +
> +    printf("Setup UUID\n");
> +    libxl_uuid_generate(&uuid);
> +    xc_domain_sethandle(dom.xch, info->domid, libxl_uuid_bytearray(&uuid));
> +
> +    printf("Creating JSON\n");
> +    rc = gen_stub_json_config(info->domid, &uuid);
> +    if (rc)
> +        err(1, "gen_stub_json_config");
> +
> +    printf("Restoring Xenstore values\n");
> +    restore_xenstore(xsh, info, uuid, dom.xenstore_evtchn);
> +
> +    printf("Introducing domain\n");
> +    xs_introduce_domain(xsh, info->domid,
> +            (GUEST_MAGIC_BASE >> XC_PAGE_SHIFT) + XENSTORE_PFN_OFFSET,
> +            dom.xenstore_evtchn);

This is really verbose and you have no option to make this a bit more quiet.
Do we really need all those prints ? Could end up being a lot if you have 5 or 6 domains.

> +    return 0;
> +}
> +
> +/* Check if domain has been configured in XS */
> +static bool domain_exists(struct xs_handle *xsh, int domid)
> +{
> +    return xs_is_domain_introduced(xsh, domid);
> +}
> +
> +int main(int argc, char **argv)
> +{
> +    libxl_dominfo *info;
> +    libxl_ctx *ctx;
> +    int nb_vm, i;
> +    struct xs_handle *xsh;
> +
> +    xsh = xs_daemon_open();
> +    if (xsh == NULL) {
> +        fprintf(stderr, "Could not contact XenStore");
> +        exit(1);
> +    }
> +
> +    if (libxl_ctx_alloc(&ctx, LIBXL_VERSION, 0, NULL)) {
> +        fprintf(stderr, "cannot init xl context\n");

You might want to close xsh to be clean here

> +        exit(1);
> +    }
> +
> +    info = libxl_list_domain(ctx, &nb_vm);
> +    if (!info) {
> +        fprintf(stderr, "libxl_list_vm failed.\n");

You might want to close xsh to be clean here

> +        exit(EXIT_FAILURE);

Why using EXIT_FAILURE here but 1 on the exit before ?

> +    }
> +
> +    for (i = 0; i < nb_vm; i++) {
> +        domid_t domid = info[i].domid;
> +
> +        /* Don't need to check for Dom0 */
> +        if (!domid)
> +            continue;
> +
> +        printf("Checking domid: %u\n", domid);
> +        if (!domain_exists(xsh, domid))
> +            init_domain(xsh, &info[i]);
> +        else
> +            printf("Domain %d has already been initialized\n", domid);
> +    }
> +    libxl_dominfo_list_free(info, nb_vm);
> +    xs_close(xsh);
> +    return 0;
> +}
> -- 
> 2.25.1
>