Add context decoupling flag.

This is a quick and dirty way to activate context decoupling related changes.
Will be removed once context decoupling is finalized.

PiperOrigin-RevId: 513854004

pkg/sentry/platform/systrap/BUILD

@@ -21,6 +21,8 @@ go_template_instance(
 go_library(
     name = "systrap",
     srcs = [
+        "context_decoupling_disable.go",
+        "context_decoupling_enable.go",
         "filters.go",
         "filters_amd64.go",
         "filters_arm64.go",

pkg/sentry/platform/systrap/context_decoupling_disable.go

@@ -0,0 +1,20 @@
+// Copyright 2023 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !context_decoupling
+// +build !context_decoupling
+
+package systrap
+
+var contextDecouplingExp bool = false

pkg/sentry/platform/systrap/context_decoupling_enable.go

@@ -0,0 +1,27 @@
+// Copyright 2023 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build context_decoupling
+// +build context_decoupling
+
+package systrap
+
+// contextDecouplingExp is a global flag that enables thread decoupling mode.
+// In this mode thread contexts are able to migrate between systrap user
+// process threads. This also allows and enables the following experimental
+// optimizations:
+//   - Ability to run M contexts using N threads, where M > N.
+//   - Reduce synchronization overhead between sentry threads and user
+//     threads when switching contexts in and out of the sentry.
+var contextDecouplingExp bool = true

pkg/sentry/platform/systrap/stub_unsafe.go

@@ -199,6 +199,10 @@ func stubInit() {
 	*p = handshakeTimeout
 	archState := (*sysmsg.ArchState)(unsafe.Pointer(stubSysmsgStart + uintptr(sysmsg.Sighandler_blob_offset____export_arch_state)))
 	archState.Init()
+	exp := (*uint64)(unsafe.Pointer(stubSysmsgStart + uintptr(sysmsg.Sighandler_blob_offset____export_context_decoupling_exp)))
+	if contextDecouplingExp {
+		*exp = 1
+	}
 
 	prepareSeccompRules(stubSysmsgStart, stubSysmsgRules, stubSysmsgRulesLen)
 
@@ -215,5 +219,5 @@ func stubInit() {
 	stubEnd = stubStart + mapLen + uintptr(gap)
 	log.Debugf("stubStart %x stubSysmsgStart %x stubSysmsgStack %x, mapLen %x", stubStart, stubSysmsgStart, stubSysmsgStack, mapLen)
 	log.Debugf(archState.String())
-
+	log.Debugf("contextDecouplingExp=%t", contextDecouplingExp)
 }

pkg/sentry/platform/systrap/sysmsg/sysmsg.h

@@ -93,6 +93,7 @@ struct sysmsg {
 extern uint64_t __export_pr_sched_core;
 extern uint64_t __export_deep_sleep_timeout;
 extern struct arch_state __export_arch_state;
+extern uint64_t __export_context_decoupling_exp;
 
 // NOLINTBEGIN(runtime/int)
 static void *sysmsg_sp() {

pkg/sentry/platform/systrap/sysmsg/sysmsg_lib.c

@@ -28,6 +28,7 @@
 uint64_t __export_deep_sleep_timeout;
 uint64_t __export_handshake_timeout;
 struct arch_state __export_arch_state;
+uint64_t __export_context_decoupling_exp;
 
 // A per-thread memory region is always align to STACK_SIZE.
 // *------------*

pkg/sentry/platform/systrap/systrap.go

@@ -374,7 +374,7 @@ func (*constructor) OpenDevice(_ string) (*os.File, error) {
 	return nil, nil
 }
 
-// Flags implements platform.Constructor.Flags().
+// Requirements implements platform.Constructor.Requirements().
 func (*constructor) Requirements() platform.Requirements {
 	// TODO(b/75837838): Also set a new PID namespace so that we limit
 	// access to other host processes.