[PATCH] KVM: x86: let it be known that ignore_msrs is a bad idea

[Paolo Bonzini <pbonzini@redhat.com>]

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");
+		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:
-- 
2.43.5