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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
0d1de2d901 ("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>
This commit is contained in:
Sean Christopherson 2024-11-27 17:34:04 -08:00
parent 136d605b43
commit 9be4ec35d6
2 changed files with 7 additions and 4 deletions
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9
Documentation/virt/kvm/api.rst
View File

@ -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
-----------------------

2
arch/x86/kvm/cpuid.c
View File

@ -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 */ |