KVM: nSVM: Advertise support for flush-by-ASID

Advertise support for FLUSHBYASID when nested SVM is enabled, as KVM can
always emulate flushing TLB entries for a vmcb12 ASID, e.g. by running L2
with a new, fresh ASID in vmcb02.  Some modern hypervisors, e.g. VMWare
Workstation 17, require FLUSHBYASID support and will refuse to run if it's
not present.

Punt on proper support, as "Honor L1's request to flush an ASID on nested
VMRUN" is one of the TODO items in the (incomplete) list of issues that
need to be addressed in order for KVM to NOT do a full TLB flush on every
nested SVM transition (see nested_svm_transition_tlb_flush()).

Reported-by: Stefan Sterz <s.sterz@proxmox.com>
Closes: https://lkml.kernel.org/r/b9915c9c-4cf6-051a-2d91-44cc6380f455%40proxmox.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20231018194104.1896415-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -5076,6 +5076,13 @@ static __init void svm_set_cpu_caps(void)
 		kvm_cpu_cap_set(X86_FEATURE_SVM);
 		kvm_cpu_cap_set(X86_FEATURE_VMCBCLEAN);
 
+		/*
+		 * KVM currently flushes TLBs on *every* nested SVM transition,
+		 * and so for all intents and purposes KVM supports flushing by
+		 * ASID, i.e. KVM is guaranteed to honor every L1 ASID flush.
+		 */
+		kvm_cpu_cap_set(X86_FEATURE_FLUSHBYASID);
+
 		if (nrips)
 			kvm_cpu_cap_set(X86_FEATURE_NRIPS);