[Xen-devel] [PATCH] evtchn: make support for different ABIs tunable

[Eslam Elnikety <elnikety@amazon.com>]

Adding support for FIFO event channel ABI was first introduced in Xen 4.4
(see 88910061ec6). Make this support tunable, since the choice of which
event channel ABI has implications for hibernation. Consider resuming a
pre Xen 4.4 hibernated Linux guest. The guest boot kernel defaults to FIFO
ABI, whereas the resume kernel assumes 2L. This, in turn, results in Xen
and the resumed kernel talking past each other (due to different protocols
FIFO vs 2L).

In order to announce to guests that the event channel ABI does not support
FIFO, the hypervisor returns ENOSYS on init_control operation. When this
operation fails, the guest should continue to use the 2L event channel ABI.
For example, in Linux drivers/xen/events/events_base.c:

    if (fifo_events)
        ret = xen_evtchn_fifo_init();
    if (ret < 0)
        xen_evtchn_2l_init();

and xen_evtchn_fifo_init fails when EVTCHNOP_init_control fails. This commit
does not change the current default behaviour: announce FIFO event channels
ABI support for guests unless explicitly stated otherwise at domaincreate.

Signed-off-by: Eslam Elnikety <elnikety@amazon.com>
---
 docs/man/xl.cfg.5.pod.in    | 5 +++++
 tools/libxl/libxl.h         | 8 ++++++++
 tools/libxl/libxl_create.c  | 5 +++++
 tools/libxl/libxl_types.idl | 1 +
 tools/xl/xl_parse.c         | 2 ++
 tools/xl/xl_sxp.c           | 2 ++
 xen/common/domain.c         | 3 +++
 xen/common/event_channel.c  | 5 +++++
 xen/include/public/domctl.h | 3 +++
 xen/include/xen/sched.h     | 1 +
 10 files changed, 35 insertions(+)

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index c99d40307e..f204d8b4f0 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -1262,6 +1262,11 @@ FIFO-based event channel ABI support up to 131,071 event channels.
 Other guests are limited to 4095 (64-bit x86 and ARM) or 1023 (32-bit
 x86).
 
+=item B<disable_evtchn_fifo=BOOLEAN>
+
+Indicates if support for FIFO event channel ABI is disabled. The default
+is false (0).
+
 =item B<vdispl=[ "VDISPL_SPEC_STRING", "VDISPL_SPEC_STRING", ...]>
 
 Specifies the virtual display devices to be provided to the guest.
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 9bacfb97f0..75b2ee3d1b 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -169,6 +169,14 @@
  */
 #define LIBXL_HAVE_BUILDINFO_EVENT_CHANNELS 1
 
+/*
+ * LIBXL_HAVE_BUILDINFO_DISABLE_EVTCHN_FIFO indicates that the
+ * libxl_domain_build_info structure contains a boolean
+ * disable_evtchn_fifo which instructs libxl to enable/disable
+ * support for FIFO event channel ABI at create time.
+ */
+#define LIBXL_HAVE_BUILDINFO_DISABLE_EVTCHN_FIFO 1
+
 /*
  * libxl_domain_build_info has the u.hvm.ms_vm_genid field.
  */
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 03ce166f4f..aa87e45643 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -217,6 +217,8 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
     if (!b_info->event_channels)
         b_info->event_channels = 1023;
 
+    libxl_defbool_setdefault(&b_info->disable_evtchn_fifo, false);
+
     libxl__arch_domain_build_info_setdefault(gc, b_info);
     libxl_defbool_setdefault(&b_info->dm_restrict, false);
 
@@ -564,6 +566,9 @@ int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
                 libxl_defbool_val(info->oos) ? 0 : XEN_DOMCTL_CDF_oos_off;
         }
 
+        if (libxl_defbool_val(b_info->disable_evtchn_fifo))
+            create.flags |= XEN_DOMCTL_CDF_disable_fifo;
+
         /* Ultimately, handle is an array of 16 uint8_t, same as uuid */
         libxl_uuid_copy(ctx, (libxl_uuid *)&create.handle, &info->uuid);
 
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index b61399ce36..5f30570443 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -521,6 +521,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
     ("iomem",            Array(libxl_iomem_range, "num_iomem")),
     ("claim_mode",	     libxl_defbool),
     ("event_channels",   uint32),
+    ("disable_evtchn_fifo", libxl_defbool),
     ("kernel",           string),
     ("cmdline",          string),
     ("ramdisk",          string),
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index e105bda2bb..bcf16c31d4 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -1496,6 +1496,8 @@ void parse_config_data(const char *config_source,
     if (!xlu_cfg_get_long(config, "max_event_channels", &l, 0))
         b_info->event_channels = l;
 
+    xlu_cfg_get_defbool(config, "disable_evtchn_fifo",
+                        &b_info->disable_evtchn_fifo, 0);
     xlu_cfg_replace_string (config, "kernel", &b_info->kernel, 0);
     xlu_cfg_replace_string (config, "ramdisk", &b_info->ramdisk, 0);
     xlu_cfg_replace_string (config, "device_tree", &b_info->device_tree, 0);
diff --git a/tools/xl/xl_sxp.c b/tools/xl/xl_sxp.c
index 359a001570..52e98c6c61 100644
--- a/tools/xl/xl_sxp.c
+++ b/tools/xl/xl_sxp.c
@@ -71,6 +71,8 @@ void printf_info_sexp(int domid, libxl_domain_config *d_config, FILE *fh)
     fprintf(fh, "\t(target_memkb %"PRId64")\n", b_info->target_memkb);
     fprintf(fh, "\t(nomigrate %s)\n",
            libxl_defbool_to_string(b_info->disable_migrate));
+    fprintf(fh, "\t(disable_evtchn_fifo %s)\n",
+           libxl_defbool_to_string(b_info->disable_evtchn_fifo));
 
     if (c_info->type == LIBXL_DOMAIN_TYPE_PV && b_info->bootloader) {
         fprintf(fh, "\t(bootloader %s)\n", b_info->bootloader);
diff --git a/xen/common/domain.c b/xen/common/domain.c
index 744b572195..d54674e28c 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -444,6 +444,9 @@ struct domain *domain_create(domid_t domid,
         d->controller_pause_count = 1;
         atomic_inc(&d->pause_count);
 
+        if ( d->options & XEN_DOMCTL_CDF_disable_fifo )
+            d->disable_evtchn_fifo = 1;
+
         if ( (err = evtchn_init(d, config->max_evtchn_port)) != 0 )
             goto fail;
         init_status |= INIT_evtchn;
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index e86e2bfab0..ce3dbb90ab 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1170,6 +1170,11 @@ long do_event_channel_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 
     case EVTCHNOP_init_control: {
         struct evtchn_init_control init_control;
+
+        /* Fail init_control for domains that must use 2l ABI */
+        if ( current->domain->disable_evtchn_fifo )
+            return -ENOSYS;
+
         if ( copy_from_guest(&init_control, arg, 1) != 0 )
             return -EFAULT;
         rc = evtchn_fifo_init_control(&init_control);
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 19486d5e32..654b4fdd22 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -64,6 +64,9 @@ struct xen_domctl_createdomain {
  /* Is this a xenstore domain? */
 #define _XEN_DOMCTL_CDF_xs_domain     4
 #define XEN_DOMCTL_CDF_xs_domain      (1U<<_XEN_DOMCTL_CDF_xs_domain)
+ /* Disable FIFO event channels? */
+#define _XEN_DOMCTL_CDF_disable_fifo  5
+#define XEN_DOMCTL_CDF_disable_fifo   (1U<<_XEN_DOMCTL_CDF_disable_fifo)
     uint32_t flags;
 
     /*
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 2e6e0d3488..fcdd802665 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -338,6 +338,7 @@ struct domain
     struct evtchn  **evtchn_group[NR_EVTCHN_GROUPS]; /* all other buckets */
     unsigned int     max_evtchns;     /* number supported by ABI */
     unsigned int     max_evtchn_port; /* max permitted port number */
+    bool             disable_evtchn_fifo;            /* force 2l ABI */
     unsigned int     valid_evtchns;   /* number of allocated event channels */
     spinlock_t       event_lock;
     const struct evtchn_port_ops *evtchn_port_ops;
-- 
2.15.3.AMZN


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