[Xen-devel] [PATCH v3] evtchn: Introduce a per-guest knob to control FIFO ABI

[Xen-devel] [PATCH v3] evtchn: Introduce a per-guest knob to control FIFO ABI

[Eslam Elnikety]

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. During resume from hibernation, there
are two kernels involved: the "boot" kernel and the "resume" kernel. The
guest boot kernel may default to FIFO ABI and instruct Xen via an
EVTCHNOP_init_control to switch from 2L to FIFO. On the other hand, the
resume kernel keeps assuming 2L. This, in turn, results in Xen and the
resumed kernel talking past each other (due to different protocols FIFO
vs 2L). A knob to control the support level for event channel ABIs per
guest would allow an administrator to work around this issue. This patch
introduces this knob.

In order to announce to guests that the event channel ABI does not support
FIFO, the hypervisor returns EPERM 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>
---
    Changes in v2:
        - Reworded the commit message
        - Return EPERM instead of ENOSYS when forcing 2L ABI
        - Use d->options instead of replicating the flag
    Changes in v3:
        - Reworded the commit subject
        - Check at evtchn_fifo_init_control, rather than EVTCHNOP_init_control
---
 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/event_channel.c  | 1 +
 xen/common/event_fifo.c     | 4 ++++
 xen/include/public/domctl.h | 3 +++
 9 files changed, 31 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..9ee80d74a6 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/event_channel.c b/xen/common/event_channel.c
index e86e2bfab0..58415e6f93 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1170,6 +1170,7 @@ long do_event_channel_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 
     case EVTCHNOP_init_control: {
         struct evtchn_init_control init_control;
+
         if ( copy_from_guest(&init_control, arg, 1) != 0 )
             return -EFAULT;
         rc = evtchn_fifo_init_control(&init_control);
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 230f440f14..3a470c97f8 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -513,6 +513,10 @@ int evtchn_fifo_init_control(struct evtchn_init_control *init_control)
     struct vcpu *v;
     int rc;
 
+    /* Fail init_control for domains that cannot use FIFO ABI */
+    if ( d->options & XEN_DOMCTL_CDF_disable_fifo )
+        return -EPERM;
+
     init_control->link_bits = EVTCHN_FIFO_LINK_BITS;
 
     vcpu_id = init_control->vcpu;
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;
 
     /*
-- 
2.15.3.AMZN


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Re: [Xen-devel] [PATCH v3] evtchn: Introduce a per-guest knob to control FIFO ABI

[Jan Beulich]

On 19.08.2019 14:08, Eslam Elnikety wrote:
> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -1170,6 +1170,7 @@ long do_event_channel_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>  
>      case EVTCHNOP_init_control: {
>          struct evtchn_init_control init_control;
> +
>          if ( copy_from_guest(&init_control, arg, 1) != 0 )
>              return -EFAULT;
>          rc = evtchn_fifo_init_control(&init_control);

Stray (leftover?) change. If intended, it should be a separate
patch dealing with all similar issues within this switch().

Apart from this cosmetic adjustment (which could be done by
the committer if no other need for a v4 arises) I can live with
this change in its current shape now, but I'm hesitant to give
my ack for the reason stated during v1/v2 review.

Jan

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