KVM: TDX: Fix x2APIC MSR handling in tdx_has_emulated_msr()

Rework tdx_has_emulated_msr() to explicitly enumerate the x2APIC MSRs
that KVM can emulate, instead of trying to enumerate the MSRs that KVM
cannot emulate. Drop the inner switch and list the emulatable x2APIC
registers directly in the outer switch's "return true" block.

The old code had multiple bugs in the x2APIC range handling.
X2APIC_MSR(APIC_ISR + APIC_ISR_NR) was incorrect because APIC_ISR_NR is
0x8, not 0x80, so the X2APIC_MSR() shift lost the lower bits, collapsing
each range to a single MSR. IA32_X2APIC_SELF_IPI was also missing from
the non-emulatable list.  Note, these bugs are relatively benign, as they
only affect a guest that is requesting "bogus" emulation.

KVM has no visibility into whether or not a guest has enabled #VE
reduction, which changes which MSRs the TDX-Module handles itself versus
triggering a #VE for the guest to make a TDVMCALL. So maintaining a list
of non-emulatable MSRs is fragile. Listing only the MSRs KVM can always
emulate sidesteps the problem.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reported-by: Dmytro Maluka <dmaluka@chromium.org>
Closes: https://lore.kernel.org/all/20260318190111.1041924-1-dmaluka@chromium.org
Fixes: dd50294f3e3c ("KVM: TDX: Implement callbacks for MSR operations")
Assisted-by: Claude:claude-opus-4-6
[based on a diff from Sean, but added missed LVTCMCI case, log]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://patch.msgid.link/20260410232654.3864196-1-rick.p.edgecombe@intel.com
[sean: call out the bugs are relatively benign, expand comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b8c3d3d8bbfe5f943329e42840c95b174f3d2b17..e58a644001861b2f5d2a7da7c1277b5b9f76a221 100644 (file)
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2116,23 +2116,29 @@ bool tdx_has_emulated_msr(u32 index)
        case MSR_IA32_MC0_CTL2 ... MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) - 1:
                /* MSR_IA32_MCx_{CTL, STATUS, ADDR, MISC, CTL2} */
        case MSR_KVM_POLL_CONTROL:
+       /*
+        * Except for x2APIC registers that are virtualized by the CPU, which
+        * KVM can't emulate as KVM doesn't have access to the virtual APIC
+        * page, KVM emulates the same set of x2APIC registers for TDX versus
+        * non-TDX guests.
+        */
+       case X2APIC_MSR(APIC_ID):
+       case X2APIC_MSR(APIC_LVR):
+       case X2APIC_MSR(APIC_LDR):
+       case X2APIC_MSR(APIC_SPIV):
+       case X2APIC_MSR(APIC_ESR):
+       case X2APIC_MSR(APIC_LVTCMCI):
+       case X2APIC_MSR(APIC_ICR):
+       case X2APIC_MSR(APIC_LVTT):
+       case X2APIC_MSR(APIC_LVTTHMR):
+       case X2APIC_MSR(APIC_LVTPC):
+       case X2APIC_MSR(APIC_LVT0):
+       case X2APIC_MSR(APIC_LVT1):
+       case X2APIC_MSR(APIC_LVTERR):
+       case X2APIC_MSR(APIC_TMICT):
+       case X2APIC_MSR(APIC_TMCCT):
+       case X2APIC_MSR(APIC_TDCR):
                return true;
-       case APIC_BASE_MSR ... APIC_BASE_MSR + 0xff:
-               /*
-                * x2APIC registers that are virtualized by the CPU can't be
-                * emulated, KVM doesn't have access to the virtual APIC page.
-                */
-               switch (index) {
-               case X2APIC_MSR(APIC_TASKPRI):
-               case X2APIC_MSR(APIC_PROCPRI):
-               case X2APIC_MSR(APIC_EOI):
-               case X2APIC_MSR(APIC_ISR) ... X2APIC_MSR(APIC_ISR + APIC_ISR_NR):
-               case X2APIC_MSR(APIC_TMR) ... X2APIC_MSR(APIC_TMR + APIC_ISR_NR):
-               case X2APIC_MSR(APIC_IRR) ... X2APIC_MSR(APIC_IRR + APIC_ISR_NR):
-                       return false;
-               default:
-                       return true;
-               }
        default:
                return false;
        }