As a mitigation for BHI, clear_bhb_loop() executes branches that overwrites
the Branch History Buffer (BHB). On Alder Lake and newer parts this
sequence is not sufficient because it doesn't clear enough entries. This
was not an issue because these CPUs have a hardware control (BHI_DIS_S)
that mitigates BHI in kernel.

BHI variant of VMSCAPE requires isolating branch history between guests and
userspace. Note that there is no equivalent hardware control for userspace.
To effectively isolate branch history on newer CPUs, clear_bhb_loop()
should execute sufficient number of branches to clear a larger BHB.

Dynamically set the loop count of clear_bhb_loop() such that it is
effective on newer CPUs too. Use the hardware control enumeration
X86_FEATURE_BHI_CTRL to select the appropriate loop count.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Nikolay Borisov <redacted>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/entry/entry_64.S   | 21 ++++++++++++++++-----
 arch/x86/net/bpf_jit_comp.c |  7 -------
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3a180a36ca0e..8128e00ca73f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S

@@ -1535,8 +1535,17 @@ SYM_CODE_END(rewind_stack_and_make_dead)
 SYM_FUNC_START(clear_bhb_loop)
        ANNOTATE_NOENDBR
        push    %rbp
+       /* BPF caller may require %rax to be preserved */
+       push    %rax