vmspawn: add --cxl= option and memory hotplug support

Add --cxl=BOOL option to enable CXL (Compute Express Link) support in
the virtual machine. CXL is a high-speed interconnect standard that
allows CPUs to access memory attached to devices such as accelerators
and memory expanders, enabling flexible memory pooling and expansion
beyond what is physically installed on the motherboard. When enabled,
adds cxl=on to the QEMU machine configuration. Only supported on x86_64
and aarch64 architectures.

This is added for testing purposes and for feature parity with mkosi's
CXL= setting.

Extend --ram= to accept an optional maximum size for memory hotplug,
using the syntax --ram=SIZE[:MAXSIZE] (e.g. --ram=2G:8G). When a
maximum is specified, the maxmem key is added to the QEMU memory
configuration section to enable memory hotplug up to the given limit.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

diff --git a/man/systemd-vmspawn.xml b/man/systemd-vmspawn.xml
index 9feb7407ca6ee20f0cb0ec2431fdb4bbc8cf8538..d65bd965a39de50824caf44a682ef14124a23300 100644 (file)
--- a/man/systemd-vmspawn.xml
+++ b/man/systemd-vmspawn.xml
@@ -167,10 +167,12 @@
         </varlistentry>
 
         <varlistentry>
-          <term><option>--ram=<replaceable>BYTES</replaceable></option></term>
+          <term><option>--ram=<replaceable>BYTES</replaceable>[:<replaceable>MAXBYTES</replaceable>[:<replaceable>SLOTS</replaceable>]]</option></term>
 
-          <listitem><para>The amount of memory to start the virtual machine with.
-          Defaults to 2G.</para>
+          <listitem><para>The amount of memory to start the virtual machine with. Defaults to 2G.
+          If a maximum size is specified after a colon, memory hotplug is enabled with the given
+          upper limit. The number of hotplug slots can optionally be specified after a second colon
+          and defaults to 1.</para>
 
           <xi:include href="version-info.xml" xpointer="v255"/>
           </listitem>
@@ -185,6 +187,18 @@
           <xi:include href="version-info.xml" xpointer="v255"/></listitem>
         </varlistentry>
 
+        <varlistentry>
+          <term><option>--cxl=<replaceable>BOOL</replaceable></option></term>
+
+          <listitem><para>Controls whether to enable CXL (Compute Express Link) support in the virtual
+          machine. CXL is a high-speed interconnect standard that allows CPUs to access memory attached to
+          devices such as accelerators and memory expanders, enabling flexible memory pooling and expansion
+          beyond what is physically installed on the motherboard. Only supported on x86_64 and aarch64
+          architectures.</para>
+
+          <xi:include href="version-info.xml" xpointer="v261"/></listitem>
+        </varlistentry>
+
         <varlistentry>
           <term><option>--vsock=<replaceable>BOOL</replaceable></option></term>
 

diff --git a/shell-completion/bash/systemd-vmspawn b/shell-completion/bash/systemd-vmspawn
index b035a42a6550ecf1868b557784a223540d8a44d8..995aeb12712987c4e59edb6e8171e8c9f6cb735b 100644 (file)
--- a/shell-completion/bash/systemd-vmspawn
+++ b/shell-completion/bash/systemd-vmspawn
@@ -31,7 +31,7 @@ _systemd_vmspawn() {
     local -A OPTS=(
         [STANDALONE]='-h --help --version -q --quiet --no-pager -n --network-tap --network-user-mode --user --system -x --ephemeral'
         [PATH]='-D --directory -i --image --linux --initrd --extra-drive --forward-journal --efi-nvram-template'
-        [BOOL]='--kvm --vsock --tpm --discard-disk --register --pass-ssh-key'
+        [BOOL]='--kvm --cxl --vsock --tpm --discard-disk --register --pass-ssh-key'
         [SECURE_BOOT]='--secure-boot'
         [FIRMWARE]='--firmware'
         [FIRMWARE_FEATURES]='--firmware-features'

diff --git a/src/vmspawn/vmspawn-util.h b/src/vmspawn/vmspawn-util.h
index 9fec6641aa3d0d75327c80de56e6fd0242a06e57..d9272b49000b22a9a1451d66ada175f281974a79 100644 (file)
--- a/src/vmspawn/vmspawn-util.h
+++ b/src/vmspawn/vmspawn-util.h
@@ -33,6 +33,12 @@
 #  define ARCHITECTURE_SUPPORTS_HPET 0
 #endif
 
+#if defined(__x86_64__) || defined(__aarch64__)
+#  define ARCHITECTURE_SUPPORTS_CXL 1
+#else
+#  define ARCHITECTURE_SUPPORTS_CXL 0
+#endif
+
 #if defined(__x86_64__) || defined(__i386__)
 #  define QEMU_MACHINE_TYPE "q35"
 #elif defined(__arm__) || defined(__aarch64__) || defined(__riscv) || defined(__loongarch64) || defined(__m68k__)

diff --git a/src/vmspawn/vmspawn.c b/src/vmspawn/vmspawn.c
index 1b1e31bb5b1034f4ecebd0305d28fd00a4a13294..92a5555f64bbcb96930e724f07d8d7f88251da92 100644 (file)
--- a/src/vmspawn/vmspawn.c
+++ b/src/vmspawn/vmspawn.c
@@ -130,7 +130,10 @@ static char *arg_slice = NULL;
 static char **arg_property = NULL;
 static char *arg_cpus = NULL;
 static uint64_t arg_ram = UINT64_C(2) * U64_GB;
+static uint64_t arg_ram_max = 0;
+static unsigned arg_ram_slots = 0;
 static int arg_kvm = -1;
+static bool arg_cxl = false;
 static int arg_vsock = -1;
 static unsigned arg_vsock_cid = VMADDR_CID_ANY;
 static int arg_tpm = -1;
@@ -229,8 +232,11 @@ static int help(void) {
                "                           scsi-cd; default: virtio-blk)\n"
                "\n%3$sHost Configuration:%4$s\n"
                "     --cpus=CPUS           Configure number of CPUs in guest\n"
-               "     --ram=BYTES           Configure guest's RAM size\n"
+               "     --ram=BYTES[:MAXBYTES[:SLOTS]]\n"
+               "                           Configure guest's RAM size (and max/slots for\n"
+               "                           hotplug)\n"
                "     --kvm=BOOL            Enable use of KVM\n"
+               "     --cxl=BOOL            Enable use of CXL\n"
                "     --vsock=BOOL          Override autodetection of VSOCK support\n"
                "     --vsock-cid=CID       Specify the CID to use for the guest's VSOCK support\n"
                "     --tpm=BOOL            Enable use of a virtual TPM\n"
@@ -326,6 +332,42 @@ static int parse_environment(void) {
         return 0;
 }
 
+static int parse_ram(const char *s) {
+        _cleanup_free_ char *ram = NULL, *ram_max = NULL, *ram_slots = NULL;
+        int r;
+
+        assert(s);
+
+        const char *p = s;
+        r = extract_many_words(&p, ":", EXTRACT_DONT_COALESCE_SEPARATORS, &ram, &ram_max, &ram_slots);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse --ram=%s: %m", s);
+        if (r == 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to parse --ram=%s", s);
+        if (!isempty(p))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Unexpected trailing data in --ram=%s", s);
+
+        r = parse_size(ram, 1024, &arg_ram);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse --ram=%s: %m", s);
+
+        if (!isempty(ram_max)) {
+                r = parse_size(ram_max, 1024, &arg_ram_max);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse --ram=%s: %m", s);
+        } else
+                arg_ram_max = 0;
+
+        if (!isempty(ram_slots)) {
+                r = safe_atou(ram_slots, &arg_ram_slots);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse --ram=%s: %m", s);
+        } else
+                arg_ram_slots = 0;
+
+        return 0;
+}
+
 static int parse_argv(int argc, char *argv[]) {
         int r;
 
@@ -340,6 +382,7 @@ static int parse_argv(int argc, char *argv[]) {
                 ARG_CPUS,
                 ARG_RAM,
                 ARG_KVM,
+                ARG_CXL,
                 ARG_VSOCK,
                 ARG_VSOCK_CID,
                 ARG_TPM,
@@ -398,6 +441,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "ram",               required_argument, NULL, ARG_RAM               },
                 { "qemu-mem",          required_argument, NULL, ARG_RAM               }, /* Compat alias */
                 { "kvm",               required_argument, NULL, ARG_KVM               },
+                { "cxl",               required_argument, NULL, ARG_CXL               },
                 { "qemu-kvm",          required_argument, NULL, ARG_KVM               }, /* Compat alias */
                 { "vsock",             required_argument, NULL, ARG_VSOCK             },
                 { "qemu-vsock",        required_argument, NULL, ARG_VSOCK             }, /* Compat alias */
@@ -518,9 +562,9 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_RAM:
-                        r = parse_size(optarg, 1024, &arg_ram);
+                        r = parse_ram(optarg);
                         if (r < 0)
-                                return log_error_errno(r, "Failed to parse --ram=%s: %m", optarg);
+                                return r;
                         break;
 
                 case ARG_KVM:
@@ -529,6 +573,15 @@ static int parse_argv(int argc, char *argv[]) {
                                 return r;
                         break;
 
+                case ARG_CXL:
+                        r = parse_boolean_argument("--cxl=", optarg, &arg_cxl);
+                        if (r < 0)
+                                return r;
+                        if (arg_cxl && !ARCHITECTURE_SUPPORTS_CXL)
+                                return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                                                       "CXL not supported on %s.", architecture_to_string(native_architecture()));
+                        break;
+
                 case ARG_VSOCK:
                         r = parse_tristate_argument_with_auto("--vsock=", optarg, &arg_vsock);
                         if (r < 0)
@@ -993,6 +1046,12 @@ static int parse_argv(int argc, char *argv[]) {
         if (!strv_isempty(arg_bind_user_groups) && strv_isempty(arg_bind_user))
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Cannot use --bind-user-group= without --bind-user=");
 
+        if (arg_ram_max > 0 && arg_ram_max < arg_ram)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Maximum RAM size must be greater than or equal to initial RAM size");
+
+        if (arg_ram_slots > 0 && arg_ram_max == 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Memory hotplug slots require a maximum RAM size");
+
         if (arg_ephemeral && arg_extra_drives.n_drives > 0)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Cannot use --ephemeral with --extra-drive=");
 
@@ -2311,6 +2370,12 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
                         return r;
         }
 
+        if (arg_cxl) {
+                r = qemu_config_key(config_file, "cxl", "on");
+                if (r < 0)
+                        return r;
+        }
+
         if (arg_directory || arg_runtime_mounts.n_mounts != 0) {
                 r = qemu_config_key(config_file, "memory-backend", "mem");
                 if (r < 0)
@@ -2333,6 +2398,16 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
         if (r < 0)
                 return r;
 
+        if (arg_ram_max > 0) {
+                r = qemu_config_keyf(config_file, "maxmem", "%" PRIu64 "M", DIV_ROUND_UP(arg_ram_max, U64_MB));
+                if (r < 0)
+                        return r;
+
+                r = qemu_config_keyf(config_file, "slots", "%u", arg_ram_slots > 0 ? arg_ram_slots : 1u);
+                if (r < 0)
+                        return r;
+        }
+
         r = qemu_config_section(config_file, "object", "rng0",
                                 "qom-type", "rng-random",
                                 "filename", "/dev/urandom");