2024-04-05 08:34:10

by Uros Bizjak

Subject: [PATCH] locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG.

Also, try_cmpxchg() implicitly assigns old *ptr value to "old"
when CMPXCHG fails. There is no need to explicitly assign
old *ptr value to the temporary, which can simplify the
surrounding source code.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
---
arch/x86/xen/p2m.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 9bdc3b656b2c..99918beccd80 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -555,7 +555,6 @@ int xen_alloc_p2m_entry(unsigned long pfn)
/* Separately check the mid mfn level */
unsigned long missing_mfn;
unsigned long mid_mfn_mfn;
- unsigned long old_mfn;

mid_mfn = alloc_p2m_page();
if (!mid_mfn)
@@ -565,12 +564,12 @@ int xen_alloc_p2m_entry(unsigned long pfn)

missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
mid_mfn_mfn = virt_to_mfn(mid_mfn);
- old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
- if (old_mfn != missing_mfn) {
- free_p2m_page(mid_mfn);
- mid_mfn = mfn_to_virt(old_mfn);
- } else {
+ /* try_cmpxchg() updates missing_mfn on failure. */
+ if (try_cmpxchg(top_mfn_p, &missing_mfn, mid_mfn_mfn)) {
p2m_top_mfn_p[topidx] = mid_mfn;
+ } else {
+ free_p2m_page(mid_mfn);
+ mid_mfn = mfn_to_virt(missing_mfn);
}
}
} else {
--
2.44.0

2024-04-11 14:00:50

by Juergen Gross

Subject: Re: [PATCH] locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

On 05.04.24 10:32, Uros Bizjak wrote:
> Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.
>
> The x86 CMPXCHG instruction returns success in the ZF flag,
> so this change saves a compare after CMPXCHG.
>
> Also, try_cmpxchg() implicitly assigns old *ptr value to "old"
> when CMPXCHG fails. There is no need to explicitly assign
> old *ptr value to the temporary, which can simplify the
> surrounding source code.
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <[email protected]>
> Cc: Juergen Gross <[email protected]>
> Cc: Boris Ostrovsky <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>

Reviewed-by: Juergen Gross <[email protected]>


Juergen