[PATCH 2/2] KVM: nSVM: Advertise support for flush-by-ASID

Sean Christopherson
2023-10-18T19:41:16Z

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
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/svm.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 1785de7dc98b..9cf7eef161ff 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5083,6 +5083,7 @@ static __init void svm_set_cpu_caps(void)
if (nested) {
kvm_cpu_cap_set(X86_FEATURE_SVM);
kvm_cpu_cap_set(X86_FEATURE_VMCBCLEAN);
+ kvm_cpu_cap_set(X86_FEATURE_FLUSHBYASID);

if (nrips)
kvm_cpu_cap_set(X86_FEATURE_NRIPS);
--
2.42.0.655.g421f12c284-goog