| Message ID | 20241219124426.325747-1-pbonzini@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: x86: let it be known that ignore_msrs is a bad idea | expand |
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@redhat.com> > --- > 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@vger.kernel.org.\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 >
On 12/20/24 18:03, Sean Christopherson wrote: > 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@redhat.com> >> --- >> 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? I don't think so, TAINT_USER seems to be for cases where there can be *host* instability. Even force_avic is a stretch. >> + 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@vger.kernel.org.\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. Will do, thanks. Paolo
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"); + 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@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(+)