Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

Pawan Gupta
2025-11-26T19:23:35Z

On Fri, Nov 21, 2025 at 08:40:44AM -0800, Dave Hansen wrote:
> On 11/19/25 22:18, Pawan Gupta wrote:
> > - CLEAR_BHB_LOOP_SEQ 5, 5
> > + /* loop count differs based on CPU-gen, see Intel's BHI guidance */
> > + ALTERNATIVE (CLEAR_BHB_LOOP_SEQ 5, 5), \
> > + __stringify(CLEAR_BHB_LOOP_SEQ 12, 7), X86_FEATURE_BHI_CTRL
>
> There are a million ways to skin this cat. But I'm not sure I really
> like the end result here. It seems a little overkill to use ALTERNATIVE
> to rewrite a whole sequence just to patch two constants in there.
>
> What if the CLEAR_BHB_LOOP_SEQ just took its inner and outer loop counts
> as register arguments? Then this would look more like:
>
> ALTERNATIVE "mov $5, %rdi; mov $5, %rsi",
> "mov $12, %rdi; mov $7, %rsi",
> ...
>
> CLEAR_BHB_LOOP_SEQ

Following this idea, loop count can be set via ALTERNATIVE within
clear_bhb_loop() itself. The outer count %ecx is already set outside the
loops. The only change to the sequence would be to also store inner count
in a register, and reload %eax from it.

---
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 886f86790b44..e4863d6d3217 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1536,7 +1536,11 @@ SYM_FUNC_START(clear_bhb_loop)
ANNOTATE_NOENDBR
push %rbp
mov %rsp, %rbp
- movl $5, %ecx
+
+ /* loop count differs based on BHI_CTRL, see Intel's BHI guidance */
+ ALTERNATIVE "movl $5, %ecx; movl $5, %edx;", \
+ "movl $12, %ecx; movl $7, %edx;", X86_FEATURE_BHI_CTRL
+
ANNOTATE_INTRA_FUNCTION_CALL
call 1f
jmp 5f
@@ -1557,7 +1561,7 @@ SYM_FUNC_START(clear_bhb_loop)
* but some Clang versions (e.g. 18) don't like this.
*/
.skip 32 - 18, 0xcc
-2: movl $5, %eax
+2: movl %edx, %eax
3: jmp 4f
nop
4: sub $1, %eax