@@ -12724,6 +12724,13 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
kvm_hv_init_vm(kvm);
kvm_xen_init_vm(kvm);
+ if (ignore_msrs && !report_ignored_msrs) {
+ pr_warn_once("Running KVM with ignore_msrs=1 and report_ignored_msrs=0 is not a\n"
+ "a supported configuration. Lying to the guest about the existence of MSRs\n"
+ "may cause the guest operating system to hang or produce errors. If a guest\n"
+ "does not run without ignore_msrs=1, please report it to kvm@vger.kernel.org.\n");
+ }
+
return 0;
out_uninit_mmu:
When running KVM with ignore_msrs=1 and report_ignored_msrs=0, the user has no clue that that the guest is being lied to. This may cause bug reports such as https://gitlab.com/qemu-project/qemu/-/issues/2571, where enabling a CPUID bit in QEMU caused Linux guests to try reading MSR_CU_DEF_ERR; and being lied about the existence of MSR_CU_DEF_ERR caused the guest to assume other things about the local APIC which were not true: Sep 14 12:02:53 kernel: mce: [Firmware Bug]: Your BIOS is not setting up LVT offset 0x2 for deferred error IRQs correctly. Sep 14 12:02:53 kernel: unchecked MSR access error: RDMSR from 0x852 at rIP: 0xffffffffb548ffa7 (native_read_msr+0x7/0x40) Sep 14 12:02:53 kernel: Call Trace: ... Sep 14 12:02:53 kernel: native_apic_msr_read+0x20/0x30 Sep 14 12:02:53 kernel: setup_APIC_eilvt+0x47/0x110 Sep 14 12:02:53 kernel: mce_amd_feature_init+0x485/0x4e0 ... Sep 14 12:02:53 kernel: [Firmware Bug]: cpu 0, try to use APIC520 (LVT offset 2) for vector 0xf4, but the register is already in use for vector 0x0 on this cpu Without reported_ignored_msrs=0 at least the host kernel log will contain enough information to avoid going on a wild goose chase. But if reports about individual MSR accesses are being silenced too, at least complain loudly the first time a VM is started. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- arch/x86/kvm/x86.c | 7 +++++++ 1 file changed, 7 insertions(+)