Move to code generation rather than reflection for io_uring offsets.

This avoids the cost at startup time and allows the resulting function to
be more effectively inlined to call sites. Although these are not widely
used at the moment, this change may also be used as an example for code
generation in similar cases.

PiperOrigin-RevId: 477597333

pkg/abi/linux/BUILD

@@ -1,4 +1,5 @@
-load("//tools:defs.bzl", "go_library", "go_test")
+load("//tools:defs.bzl", "arch_genrule", "go_library", "go_test")
+load("//tools/nogo:defs.bzl", "nogo_facts")
 
 # Package linux contains the constants and types needed to interface with a
 # Linux kernel. It should be used instead of syscall or golang.org/x/sys/unix
@@ -6,6 +7,19 @@ load("//tools:defs.bzl", "go_library", "go_test")
 
 package(licenses = ["notice"])
 
+nogo_facts(
+    name = "iouring_offsets",
+    srcs = ["iouring.go"],
+    output = "iouring_offsets_impl.go",
+    template = "iouring_offsets.tmpl",
+)
+
+arch_genrule(
+    name = "iouring_offsets_arch",
+    src = ":iouring_offsets",
+    template = "iouring_offsets_%s.go",
+)
+
 go_library(
     name = "linux",
     srcs = [
@@ -74,6 +88,7 @@ go_library(
         "utsname.go",
         "wait.go",
         "xattr.go",
+        ":iouring_offsets_arch",
     ],
     marshal = True,
     visibility = ["//visibility:public"],

pkg/abi/linux/iouring.go

@@ -14,11 +14,6 @@
 
 package linux
 
-import (
-	"fmt"
-	"reflect"
-)
-
 // Constants for io_uring_setup(2). See include/uapi/linux/io_uring.h.
 const (
 	IORING_SETUP_IOPOLL     = (1 << 0)
@@ -156,36 +151,3 @@ type IORings struct {
 	// Linux has an additional field struct io_uring_cqe cqes[], which represents
 	// a dynamic array. We don't include it here in order to enable marshalling.
 }
-
-// PreComputedIOSqRingOffsets stores precomputed values for IOSqRingOffsets.
-var PreComputedIOSqRingOffsets IOSqRingOffsets
-
-// PreComputedIOCqRingOffsets stores precomputed values for IOCqRingOffsets.
-var PreComputedIOCqRingOffsets IOCqRingOffsets
-
-func init() {
-	ioRingsType := reflect.TypeOf((*IORings)(nil)).Elem()
-	ioUringType := reflect.TypeOf((*IOUring)(nil)).Elem()
-
-	offsetof := func(ty reflect.Type, name string) uint32 {
-		if f, ok := ty.FieldByName(name); ok {
-			return uint32(f.Offset)
-		}
-		panic(fmt.Sprintf("In type %q, no field named %q", ty.Name(), name))
-	}
-
-	PreComputedIOSqRingOffsets.Head = offsetof(ioRingsType, "sq") + offsetof(ioUringType, "head")
-	PreComputedIOSqRingOffsets.Tail = offsetof(ioRingsType, "sq") + offsetof(ioUringType, "tail")
-	PreComputedIOSqRingOffsets.RingMask = offsetof(ioRingsType, "sqRingMask")
-	PreComputedIOSqRingOffsets.RingEntries = offsetof(ioRingsType, "sqRingEntries")
-	PreComputedIOSqRingOffsets.Flags = offsetof(ioRingsType, "sqFlags")
-	PreComputedIOSqRingOffsets.Dropped = offsetof(ioRingsType, "sqDropped")
-
-	PreComputedIOCqRingOffsets.Head = offsetof(ioRingsType, "cq") + offsetof(ioUringType, "head")
-	PreComputedIOCqRingOffsets.Tail = offsetof(ioRingsType, "cq") + offsetof(ioUringType, "tail")
-	PreComputedIOCqRingOffsets.RingMask = offsetof(ioRingsType, "cqRingMask")
-	PreComputedIOCqRingOffsets.RingEntries = offsetof(ioRingsType, "cqRingEntries")
-	PreComputedIOCqRingOffsets.Overflow = offsetof(ioRingsType, "cqOverflow")
-	PreComputedIOCqRingOffsets.Flags = offsetof(ioRingsType, "cqFlags")
-
-}

pkg/abi/linux/iouring_offsets.tmpl

@@ -0,0 +1,39 @@
+// Copyright 2022 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.
+
+package linux
+
+// PreComputedIOSqRingOffsets returns precomputed values for IOSqRingOffsets.
+func PreComputedIOSqRingOffsets() IOSqRingOffsets {
+	return IOSqRingOffsets{
+		Head: {{ .IORings.sq.Offset }} + {{ .IOUring.head.Offset }},
+		Tail: {{ .IORings.sq.Offset }} + {{ .IOUring.tail.Offset }},
+		RingMask: {{ .IORings.sqRingMask.Offset }},
+		RingEntries: {{ .IORings.sqRingEntries.Offset }},
+		Flags: {{ .IORings.sqFlags.Offset }},
+		Dropped: {{ .IORings.sqDropped.Offset }},
+	}
+}
+
+// PreComputedIOCqRingOffsets returns precomputed values for IOCqRingOffsets.
+func PreComputedIOCqRingOffsets() IOCqRingOffsets {
+	return IOCqRingOffsets {
+		Head: {{ .IORings.cq.Offset }} + {{ .IOUring.head.Offset }},
+		Tail: {{ .IORings.cq.Offset }} + {{ .IOUring.tail.Offset }},
+		RingMask: {{ .IORings.cqRingMask.Offset }},
+		RingEntries: {{ .IORings.cqRingEntries.Offset }},
+		Overflow: {{ .IORings.cqOverflow.Offset }},
+		Flags: {{ .IORings.cqFlags.Offset }},
+	}
+}

pkg/sentry/fsimpl/iouringfs/iouringfs.go

@@ -137,7 +137,7 @@ func New(ctx context.Context, vfsObj *vfs.VirtualFilesystem, entries uint32, par
 		return nil, linuxerr.EOVERFLOW
 	}
 
-	params.SqOff = linux.PreComputedIOSqRingOffsets
+	params.SqOff = linux.PreComputedIOSqRingOffsets()
 	params.SqOff.Array = uint32(arrayOffset)
 
 	cqesOffset := uint64(hostarch.Addr((*linux.IORings)(nil).SizeBytes()))
@@ -146,7 +146,7 @@ func New(ctx context.Context, vfsObj *vfs.VirtualFilesystem, entries uint32, par
 		return nil, linuxerr.EOVERFLOW
 	}
 
-	params.CqOff = linux.PreComputedIOCqRingOffsets
+	params.CqOff = linux.PreComputedIOCqRingOffsets()
 	params.CqOff.Cqes = uint32(cqesOffset)
 
 	// Set features supported by the current IO_URING implementation.