git: 6fa9c2b1d282 - main - arm64: close a race in SVE register management

The branch main has been updated by alc:

URL: https://cgit.FreeBSD.org/src/commit/?id=6fa9c2b1d2824034268646e0744e5e010155ffaf

commit 6fa9c2b1d2824034268646e0744e5e010155ffaf
Author:     Alan Cox <alc@FreeBSD.org>
AuthorDate: 2026-08-07 21:51:04 +0000
Commit:     Alan Cox <alc@FreeBSD.org>
CommitDate: 2026-08-14 20:57:35 +0000

    arm64: close a race in SVE register management
    
    While testing an unrelated pmap change, D58708, that dramatically
    reduces the number of TLBI instructions performed, and likely the
    timing of unrelated events, I started seeing "Storing an invalid VFP
    state" panics in vfp_save_state_common().  However, the origin of this
    panic is elsewhere, in the else branch of sve_restore_state().
    Specifically, my pmap change seems to have increased the likelihood that
    the thread executing the else branch would be preempted by another
    thread between the critical_exit() inside the else branch's call to
    vfp_restore_state_common() and its own call to critical_enter().  Prior
    to expanding the scope of the else branch's critical section, the MPASS
    added by this change would fire, catching the problem at its source,
    rather than later in vfp_save_state_common().
    
    Assisted-by:    Claude Code (Opus 5)
    Reviewed by:    kib, markj
    MFC after:      1 week
    Differential Revision:  https://reviews.freebsd.org/D58723
---
 sys/arm64/arm64/vfp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sys/arm64/arm64/vfp.c b/sys/arm64/arm64/vfp.c
index d4c0eacf690c..11faa4cc21cd 100644
--- a/sys/arm64/arm64/vfp.c
+++ b/sys/arm64/arm64/vfp.c
@@ -797,15 +797,17 @@ sve_restore_state(struct thread *td)
 
 		critical_exit();
 	} else {
+		critical_enter();
+
 		vfp_restore_state_common(td, curpcb->pcb_fpflags);
 
 		/* Enable SVE if it wasn't previously enabled */
 		if ((curpcb->pcb_fpflags & PCB_FP_SVEVALID) == 0) {
-			critical_enter();
+			MPASS(PCPU_GET(fpcurthread) == td);
 			sve_enable();
 			curpcb->pcb_fpflags |= PCB_FP_SVEVALID;
-			critical_exit();
 		}
+		critical_exit();
 	}
 
 	return (true);