[src/trunk]: src/sys/arch/x86/x86 Limit x86 fpu_kern_enter/leave to IPL_VM or...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/3e69022c9e64
branches:  trunk
changeset: 1011852:3e69022c9e64
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Mon Jul 13 16:51:51 2020 +0000

description:
Limit x86 fpu_kern_enter/leave to IPL_VM or below.

There are no users of crypto at IPL_SCHED or IPL_HIGH as far as I
know, and although we generally limit the amount of time spent in any
one crypto operation -- e.g., cgd is usually limited to processing
512 or 4096 bytes at a time -- it's better not to block IPL_SCHED and
IPL_HIGH interrupts at all.  This should make ddb a little more
accessible during crypto-heavy workloads.

This means the aes_* API cannot be used at IPL_SCHED or IPL_HIGH; the
same will go for any new crypto subsystems, like the ChaCha and
Poly1305 ones I'm drafting.  It might be better to prohibit them
altogether in hard interrupt context, but right now cprng_fast and
cprng_strong are both technically allowed at IPL_VM and are sometimes
used there (e.g., for opencrypto CBC IV generation).

KASSERT the ilevel to detect violation of this constraint in case I'm
wrong.

diffstat:

 sys/arch/x86/x86/fpu.c |  14 +++++++++-----
 1 files changed, 9 insertions(+), 5 deletions(-)

diffs (55 lines):

diff -r 1aec40194e36 -r 3e69022c9e64 sys/arch/x86/x86/fpu.c
--- a/sys/arch/x86/x86/fpu.c    Mon Jul 13 15:05:05 2020 +0000
+++ b/sys/arch/x86/x86/fpu.c    Mon Jul 13 16:51:51 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: fpu.c,v 1.67 2020/07/06 18:30:48 riastradh Exp $       */
+/*     $NetBSD: fpu.c,v 1.68 2020/07/13 16:51:51 riastradh Exp $       */
 
 /*
  * Copyright (c) 2008, 2019 The NetBSD Foundation, Inc.  All
@@ -96,7 +96,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.67 2020/07/06 18:30:48 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.68 2020/07/13 16:51:51 riastradh Exp $");
 
 #include "opt_multiprocessor.h"
 
@@ -370,11 +370,14 @@
 /*
  * fpu_kern_enter()
  *
- *     Begin using the FPU.  Raises to splhigh, disabling all
+ *     Begin using the FPU.  Raises to splvm, disabling most
  *     interrupts and rendering the thread non-preemptible; caller
  *     should not use this for long periods of time, and must call
  *     fpu_kern_leave() afterward.  Non-recursive -- you cannot call
  *     fpu_kern_enter() again without calling fpu_kern_leave() first.
+ *
+ *     Must be used only at IPL_VM or below -- never in IPL_SCHED or
+ *     IPL_HIGH interrupt handlers.
  */
 void
 fpu_kern_enter(void)
@@ -383,9 +386,10 @@
        struct cpu_info *ci;
        int s;
 
-       s = splhigh();
+       s = splvm();
 
        ci = curcpu();
+       KASSERTMSG(ci->ci_ilevel <= IPL_VM, "ilevel=%d", ci->ci_ilevel);
        KASSERT(ci->ci_kfpu_spl == -1);
        ci->ci_kfpu_spl = s;
 
@@ -423,7 +427,7 @@
        struct cpu_info *ci = curcpu();
        int s;
 
-       KASSERT(ci->ci_ilevel == IPL_HIGH);
+       KASSERT(ci->ci_ilevel == IPL_VM);
        KASSERT(ci->ci_kfpu_spl != -1);
 
        /*