From cee4007b0108adc469a4e51394f83878a35e067a Mon Sep 17 00:00:00 2001
From: Leon Hwang <leon.hwang@linux.dev>
Date: Wed, 1 Oct 2025 17:06:08 +0800
Subject: [PATCH] [qemu] Add custom kvm cpu args support

When improve `bpf_get_branch_snapshot()` helper, it requires passing LBR
into the VM like `-cpu,host,pmu=on,lbr-fmt=0x5`.

Then, in the VM, confirm the LBR is supported:

```bash
root@(none):/# dmesg | grep -i lbr
[    0.394406] Performance Events: Skylake events, 32-deep LBR, full-width counters, Intel PMU driver.
```

With this commit, the VM can be created by
`vmtest -k $(make -s image_name) --kvm-cpu-args 'host,pmu=on,lbr-fmt=0x5' -`.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 src/config.rs | 10 ++++++++++
 src/main.rs   |  4 ++++
 src/qemu.rs   | 17 ++++++++++++-----
 3 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/config.rs b/src/config.rs
index 83a49a2..1b58fac 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -100,6 +100,11 @@ pub struct Target {
     ///
     /// Arguments are only valid for kernel targets.
     pub kernel_args: Option<String>,
+    /// KVM -cpu arguments are only valid for KVM enabled targets.
+    ///
+    /// Default: host
+    #[serde(default = "Target::default_kvm_cpu_args")]
+    pub kvm_cpu_args: Option<String>,
     /// Path to rootfs to test against.
     ///
     /// * The path is relative to `vmtest.toml`.
@@ -130,6 +135,10 @@ impl Target {
     pub fn default_arch() -> String {
         ARCH.to_string()
     }
+    /// Default kvm cpu args to use if none are specified.
+    pub fn default_kvm_cpu_args() -> Option<String> {
+        Some("host".into())
+    }
 }
 
 impl Default for Target {
@@ -140,6 +149,7 @@ impl Default for Target {
             uefi: false,
             kernel: None,
             kernel_args: None,
+            kvm_cpu_args: None,
             rootfs: Self::default_rootfs(),
             arch: Self::default_arch(),
             qemu_command: None,
diff --git a/src/main.rs b/src/main.rs
index 3e705eb..a2227f5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -37,6 +37,9 @@ struct Args {
     /// Additional kernel command line arguments
     #[clap(long, conflicts_with = "config")]
     kargs: Option<String>,
+    /// KVM -cpu arguments
+    #[clap(long, conflicts_with = "config", default_value = "host")]
+    kvm_cpu_args: Option<String>,
     /// Location of rootfs, default to host's /
     #[clap(short, long, conflicts_with = "config", default_value = Target::default_rootfs().into_os_string())]
     rootfs: PathBuf,
@@ -122,6 +125,7 @@ fn config(args: &Args) -> Result<Vmtest> {
                     rootfs: args.rootfs.clone(),
                     arch: args.arch.clone(),
                     kernel_args: args.kargs.clone(),
+                    kvm_cpu_args: args.kvm_cpu_args.clone(),
                     qemu_command: args.qemu_command.clone(),
                     command: args.command.join(" "),
                     vm: VMConfig::default(),
diff --git a/src/qemu.rs b/src/qemu.rs
index 9758110..627a2d6 100644
--- a/src/qemu.rs
+++ b/src/qemu.rs
@@ -251,13 +251,17 @@ fn guest_agent_args(sock: &Path) -> Vec<OsString> {
 }
 
 /// Generate arguments for full KVM virtualization if host supports it
-fn kvm_args(arch: &str) -> Vec<&'static str> {
+fn kvm_args<'a>(arch: &str, kvm_cpu_args: &'a str) -> Vec<&'a str> {
     let mut args = Vec::new();
 
     if host_supports_kvm(arch) {
         args.push("-enable-kvm");
         args.push("-cpu");
-        args.push("host");
+        if kvm_cpu_args.is_empty() {
+            args.push("host");
+        } else {
+            args.push(kvm_cpu_args);
+        }
     } else {
         args.push("-cpu");
         match arch {
@@ -365,7 +369,7 @@ fn kernel_args(
     kernel: &Path,
     arch: &str,
     init: &Path,
-    additional_kargs: Option<&String>,
+    additional_kargs: Option<&str>,
 ) -> Vec<OsString> {
     let mut args = Vec::new();
 
@@ -669,7 +673,10 @@ impl Qemu {
             .stderr(Stdio::piped())
             .arg("-serial")
             .arg("mon:stdio")
-            .args(kvm_args(&target.arch))
+            .args(kvm_args(
+                &target.arch,
+                target.kvm_cpu_args.as_deref().unwrap_or("host"),
+            ))
             .args(machine_args(&target.arch))
             .args(machine_protocol_args(&qmp_sock))
             .args(guest_agent_args(&qga_sock))
@@ -691,7 +698,7 @@ impl Qemu {
                 kernel,
                 &target.arch,
                 guest_init.as_path(),
-                target.kernel_args.as_ref(),
+                target.kernel_args.as_deref(),
             ));
         } else {
             panic!("Config validation should've enforced XOR");