pvmforwardport.patch

From 74ec385f298c3a6811a1f1f3aa119b166782877f Mon Sep 17 00:00:00 2001
From: PVM Forward-Port <pvm@local>
Date: Tue, 23 Jun 2026 15:01:44 +0800
Subject: [PATCH] PVM forward-port: preserve loopholelabs PVM changes on modern
 upstream

Re-applies the loopholelabs (PVM / no-KVM Firecracker) source changes on top
of a current upstream Firecracker checkout, so the fork can be updated to the
latest upstream WITHOUT hand-merging or losing the PVM behaviour.

Two changes are required; everything else PVM needs is already satisfied by
modern upstream (see notes at the bottom of this patch):

1. vcpu.rs - make set_tsc_khz() a no-op.
   PVM does not support KVM TSC scaling (KVM_SET_TSC_KHZ). Upstream calls it
   unconditionally on snapshot restore, which fails / can freeze the guest on
   PVM. We drop the ioctl and return Ok(()).

2. custom_cpu_template.rs - relax the static CPU-template model check.
   The PVM vCPU does not match the host CPU model table, so the strict
   InvalidCpuModel guard rejects templates like T2A. We disable just that
   check (the vendor check is kept).

Apply with either:
    git apply pvm-forward-port.patch        # just the working-tree changes
    git am    pvm-forward-port.patch        # as a commit (preserves this message)

If upstream context has drifted, fall back to a fuzzy apply:
    git apply --3way pvm-forward-port.patch
    # or:  patch -p1 --fuzz=3 < pvm-forward-port.patch

NOTE on MSRs: the fork also added explicit PVM MSR entries in msr.rs. Modern
upstream replaced the hand-listed MSR allow-list with a RANGE-based
serialization scheme that already covers the PVM MSR range, so no msr.rs change
is needed on current upstream. If you target an older upstream that still uses
the explicit list, add the PVM MSRs (0x4b564d00..=0x4b564dff region) there too.
---
 src/vmm/src/arch/x86_64/vcpu.rs                      | 8 ++++++--
 src/vmm/src/cpu_config/x86_64/custom_cpu_template.rs | 3 ++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/vmm/src/arch/x86_64/vcpu.rs b/src/vmm/src/arch/x86_64/vcpu.rs
index 98563aa..5e4c846 100644
--- a/src/vmm/src/arch/x86_64/vcpu.rs
+++ b/src/vmm/src/arch/x86_64/vcpu.rs
@@ -640,8 +640,12 @@ impl KvmVcpu {
     }
 
     /// Scale the TSC frequency of this vCPU to the one provided as a parameter.
-    pub fn set_tsc_khz(&self, tsc_freq: u32) -> Result<(), SetTscError> {
-        self.fd.set_tsc_khz(tsc_freq).map_err(SetTscError)
+    pub fn set_tsc_khz(&self, _: u32) -> Result<(), SetTscError> {
+        // Disable TSC scaling when using PVM because it is unsupported on most virtualized platforms.
+        // Even on supported platforms like virtualized AMD CPUs, enabling TSC scaling can cause VM freezes
+        // after resuming from a snapshot.
+        // For more details, see https://github.com/virt-pvm/linux/issues/12#issue-2515360332
+        Ok(())
     }
 
     /// Use provided state to populate KVM internal state.
diff --git a/src/vmm/src/cpu_config/x86_64/custom_cpu_template.rs b/src/vmm/src/cpu_config/x86_64/custom_cpu_template.rs
index 1e790f1..f01a0a2 100644
--- a/src/vmm/src/cpu_config/x86_64/custom_cpu_template.rs
+++ b/src/vmm/src/cpu_config/x86_64/custom_cpu_template.rs
@@ -39,7 +39,8 @@ impl GetCpuTemplate for Option<CpuTemplateType> {
 
                     let cpu_model = CpuModel::get_cpu_model();
                     if !template.get_supported_cpu_models().contains(&cpu_model) {
-                        return Err(InvalidCpuModel);
+                        // Disable the CPU compatibility check to allow using templates like T2A on more modern CPUs
+                        // return Err(InvalidCpuModel);
                     }
 
                     match template {
-- 
2.52.0