KVM: x86: Advertise TSC_DEADLINE_TIMER in KVM_GET_SUPPORTED_CPUID

Unconditionally advertise TSC_DEADLINE_TIMER via KVM_GET_SUPPORTED_CPUID,
as KVM always emulates deadline mode, *if* the VM has an in-kernel local
APIC.  The odds of a VMM emulating the local APIC in userspace, not
emulating the TSC deadline timer, _and_ reflecting
KVM_GET_SUPPORTED_CPUID back into KVM_SET_CPUID2, i.e. the risk of
over-advertising and breaking any setups, is extremely low.

KVM has _unconditionally_ advertised X2APIC via CPUID since commit
0d1de2d901f4 ("KVM: Always report x2apic as supported feature"), and it
is completely impossible for userspace to emulate X2APIC as KVM doesn't
support forwarding the MSR accesses to userspace.  I.e. KVM has relied on
userspace VMMs to not misreport local APIC capabilities for nearly 13
years.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-38-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index bbe445e6c1132f1147263cca18161de75989bed9..61bf1f693e2d13f556a1cee9cf04bf7173432cd6 100644 (file)
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1825,15 +1825,18 @@ emulate them efficiently. The fields in each entry are defined as follows:
          the values returned by the cpuid instruction for
          this function/index combination
 
-The TSC deadline timer feature (CPUID leaf 1, ecx[24]) is always returned
-as false, since the feature depends on KVM_CREATE_IRQCHIP for local APIC
-support.  Instead it is reported via::
+x2APIC (CPUID leaf 1, ecx[21) and TSC deadline timer (CPUID leaf 1, ecx[24])
+may be returned as true, but they depend on KVM_CREATE_IRQCHIP for in-kernel
+emulation of the local APIC.  TSC deadline timer support is also reported via::
 
   ioctl(KVM_CHECK_EXTENSION, KVM_CAP_TSC_DEADLINE_TIMER)
 
 if that returns true and you use KVM_CREATE_IRQCHIP, or if you emulate the
 feature in userspace, then you can enable the feature for KVM_SET_CPUID2.
 
+Enabling x2APIC in KVM_SET_CPUID2 requires KVM_CREATE_IRQCHIP as KVM doesn't
+support forwarding x2APIC MSR accesses to userspace, i.e. KVM does not support
+emulating x2APIC in userspace.
 
 4.47 KVM_PPC_GET_PVINFO
 -----------------------

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 922ad598787a3c827e6d1ca88691cdd9257e39f8..f0ee773704d5edc1f9cc0a6f0c2c53c6237e592a 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -737,7 +737,7 @@ void kvm_set_cpu_caps(void)
                EMULATED_F(X2APIC) |
                F(MOVBE) |
                F(POPCNT) |
-               0 /* Reserved*/ |
+               EMULATED_F(TSC_DEADLINE_TIMER) |
                F(AES) |
                F(XSAVE) |
                0 /* OSXSAVE */ |