KVM: SVM: Fix redundant updates of LBR MSR intercepts

KVM: SVM: Fix redundant updates of LBR MSR intercepts

Don't update the LBR MSR intercept bitmaps if they're already up-to-date,
as unconditionally updating the intercepts forces KVM to recalculate the
MSR bitmaps for vmcb02 on every nested VMRUN.  The redundant updates are
functionally okay; however, they neuter an optimization in Hyper-V
nested virtualization enlightenments and this manifests as a self-test
failure.

In particular, Hyper-V lets L1 mark "nested enlightenments" as clean, i.e.
tell KVM that no changes were made to the MSR bitmap since the last VMRUN.
The hyperv_svm_test KVM selftest intentionally changes the MSR bitmap
"without telling KVM about it" to verify that KVM honors the clean hint,
correctly fails because KVM notices the changed bitmap anyway:

  ==== Test Assertion Failure ====
  x86/hyperv_svm_test.c:120: vmcb->control.exit_code == 0x081
  pid=193558 tid=193558 errno=4 - Interrupted system call
     1	0x0000000000411361: assert_on_unhandled_exception at processor.c:659
     2	0x0000000000406186: _vcpu_run at kvm_util.c:1699
     3	 (inlined by) vcpu_run at kvm_util.c:1710
     4	0x0000000000401f2a: main at hyperv_svm_test.c:175
     5	0x000000000041d0d3: __libc_start_call_main at libc-start.o:?
     6	0x000000000041f27c: __libc_start_main_impl at ??:?
     7	0x00000000004021a0: _start at ??:?
  vmcb->control.exit_code == SVM_EXIT_VMMCALL

Do *not* fix this by skipping svm_hv_vmcb_dirty_nested_enlightenments()
when svm_set_intercept_for_msr() performs a no-op change.  changes to
the L0 MSR interception bitmap are only triggered by full CPUID updates
and MSR filter updates, both of which should be rare.  Changing
svm_set_intercept_for_msr() risks hiding unintended pessimizations
like this one, and is actually more complex than this change.

Fixes: fbe5e5f030c2 ("KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()")
Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251112013017.1836863-1-yosry.ahmed@linux.dev
[Rewritten commit message based on mailing list discussion. - Paolo]
Reviewed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>