15254 %ymm registers not restored after signal handler
15367 x86 getfpregs() summons corrupting %xmm ghosts
15333 want x86 /proc xregs support (libc_db, libproc, mdb, etc.)
15336 want libc functions for extended ucontext_t
15334 want ps_lwphandle-specific reg routines
15328 FPU_CW_INIT mistreats reserved bit
15335 i86pc fpu_subr.c isn't really platform-specific
15332 setcontext(2) isn't actually noreturn
15331 need <sys/stdalign.h>
Change-Id: I7060aa86042dfb989f77fc3323c065ea2eafa9ad
Conflicts:
    usr/src/uts/common/fs/proc/prcontrol.c
    usr/src/uts/intel/os/archdep.c
    usr/src/uts/intel/sys/ucontext.h
    usr/src/uts/intel/syscall/getcontext.c

@@ -23,11 +23,11 @@
  * Copyright (c) 1989, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright 2019 Joyent, Inc.
  * Copyright (c) 2017 by Delphix. All rights reserved.
  * Copyright 2020 OmniOS Community Edition (OmniOSce) Association.
  * Copyright 2022 MNX Cloud, Inc.
- * Copyright 2022 Oxide Computer Company
+ * Copyright 2023 Oxide Computer Company
  */
 
 /*      Copyright (c) 1984,      1986, 1987, 1988, 1989 AT&T    */
 /*        All Rights Reserved   */
 

@@ -1735,28 +1735,48 @@
 
 /* ARGSUSED */
 static int
 pr_read_xregs(prnode_t *pnp, uio_t *uiop, cred_t *cr)
 {
-#if defined(__sparc)
         proc_t *p;
         kthread_t *t;
         int error;
-        char *xreg;
+        void *xreg;
         size_t size;
 
         ASSERT(pnp->pr_type == PR_XREGS);
 
-        xreg = kmem_zalloc(sizeof (prxregset_t), KM_SLEEP);
-
         if ((error = prlock(pnp, ZNO)) != 0)
-                goto out;
+                return (error);
 
         p = pnp->pr_common->prc_proc;
         t = pnp->pr_common->prc_thread;
 
-        size = prhasx(p)? prgetprxregsize(p) : 0;
+        /*
+         * While we would prefer to do the allocation with holding the process
+         * this way, we can only determine this size while holding the process
+         * as the hold guarantees us:
+         *
+         *  o That the process in question actualy exists.
+         *  o That the process in question cannot change the set of FPU features
+         *    it has enabled.
+         *
+         * We will drop p_lock across the allocation call itself. This should be
+         * safe as the enabled feature set should not change while the process
+         * is locked (e.g. enabling extending FPU state like AMX on x86 should
+         * require the process to be locked).
+         */
+        size = prhasx(p) ? prgetprxregsize(p) : 0;
+        if (size == 0) {
+                prunlock(pnp);
+                return (0);
+        }
+        mutex_exit(&p->p_lock);
+        xreg = kmem_zalloc(size, KM_SLEEP);
+        mutex_enter(&p->p_lock);
+        ASSERT3U(size, ==, prgetprxregsize(p));
+
         if (uiop->uio_offset >= size) {
                 prunlock(pnp);
                 goto out;
         }
 

@@ -1766,15 +1786,12 @@
         mutex_enter(&p->p_lock);
         prunlock(pnp);
 
         error = pr_uioread(xreg, size, uiop);
 out:
-        kmem_free(xreg, sizeof (prxregset_t));
+        kmem_free(xreg, size);
         return (error);
-#else
-        return (0);
-#endif
 }
 
 static int
 pr_read_spymaster(prnode_t *pnp, uio_t *uiop, cred_t *cr)
 {