From f193bca595c0050474c4f1f4e4540151f67e4250 Mon Sep 17 00:00:00 2001
From: Bernardo Meurer Costa <beme@anthropic.com>
Date: Wed, 27 Aug 2025 22:19:07 +0000
Subject: [PATCH] feat(libstore): warn when kvm is enabled but /dev/kvm isn't
 available

---
 src/libstore/unix/build/linux-derivation-builder.cc | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/libstore/unix/build/linux-derivation-builder.cc b/src/libstore/unix/build/linux-derivation-builder.cc
index b92d05607..3e34a1a7f 100644
--- a/src/libstore/unix/build/linux-derivation-builder.cc
+++ b/src/libstore/unix/build/linux-derivation-builder.cc
@@ -3,6 +3,7 @@
 #  include "nix/store/personality.hh"
 #  include "nix/util/cgroup.hh"
 #  include "nix/util/linux-namespaces.hh"
+#  include "nix/util/logging.hh"
 #  include "linux/fchmodat2-compat.hh"
 
 #  include <sys/ioctl.h>
@@ -492,8 +493,16 @@ struct ChrootLinuxDerivationBuilder : ChrootDerivationBuilder, LinuxDerivationBu
             createDirs(chrootRootDir + "/dev/shm");
             createDirs(chrootRootDir + "/dev/pts");
             ss.push_back("/dev/full");
-            if (systemFeatures.count("kvm") && pathExists("/dev/kvm"))
-                ss.push_back("/dev/kvm");
+            if (systemFeatures.count("kvm")) {
+                if (pathExists("/dev/kvm")) {
+                    ss.push_back("/dev/kvm");
+                } else {
+                    warn(
+                        "KVM is enabled in system-features but /dev/kvm is not available. "
+                        "QEMU builds may fall back to slow emulation. "
+                        "Consider removing 'kvm' from system-features in nix.conf if KVM is not supported on this system.");
+                }
+            }
             ss.push_back("/dev/null");
             ss.push_back("/dev/random");
             ss.push_back("/dev/tty");