On Thu, Dec 19, 2024, Paolo Bonzini wrote: > 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@xxxxxxxxxx> > --- > arch/x86/kvm/x86.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index c8160baf3838..1b7c8db0cf63 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -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"); > + pr_warn_once("a supported configuration. Lying to the guest about the existence of MSRs\n"); Back-to-back 'a's. If we're saying this combo is unsupported, should we taint the host kernel with TAINT_USER, e.g. similar to how the force_avic parameter is treated as unsafe? > + pr_warn_once("may cause the guest operating system to hang or produce errors. If a guest\n"); > + pr_warn_once("does not run without ignore_msrs=1, please report it to kvm@xxxxxxxxxxxxxxx.\n"); This should be a multi-line string that's printed in a single pr_warn_once(), otherwise the full message could get split quite weirdly if there is other dmesg activity. > + } > + > return 0; > > out_uninit_mmu: > -- > 2.43.5 >
