Lower the amd64 shared page, which contains the signal trampoline,

from the top of user memory to one page lower on machines with the
Ryzen (AMD Family 17h) CPU.  This pushes ps_strings and the stack
down by one page as well.  On Ryzen there is some sort of interaction
between code running at the top of user memory address space and
interrupts that can cause FreeBSD to either hang or silently reset.
This sounds similar to the problem found with DragonFly BSD that
was fixed with this commit:
  https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/b48dd28447fc8ef62fbc963accd301557fd9ac20
but our signal trampoline location was already lower than the address
that DragonFly moved their signal trampoline to.  It also does not
appear to be related to SMT as described here:
  https://www.phoronix.com/forums/forum/hardware/processors-memory/955368-some-ryzen-linux-users-are-facing-issues-with-heavy-compilation-loads?p=955498#post955498

  "Hi, Matt Dillon here. Yes, I did find what I believe to be a
   hardware issue with Ryzen related to concurrent operations. In a
   nutshell, for any given hyperthread pair, if one hyperthread is
   in a cpu-bound loop of any kind (can be in user mode), and the
   other hyperthread is returning from an interrupt via IRETQ, the
   hyperthread issuing the IRETQ can stall indefinitely until the
   other hyperthread with the cpu-bound loop pauses (aka HLT until
   next interrupt). After this situation occurs, the system appears
   to destabilize. The situation does not occur if the cpu-bound
   loop is on a different core than the core doing the IRETQ. The
   %rip the IRETQ returns to (e.g. userland %rip address) matters a
   *LOT*. The problem occurs more often with high %rip addresses
   such as near the top of the user stack, which is where DragonFly's
   signal trampoline traditionally resides. So a user program taking
   a signal on one thread while another thread is cpu-bound can cause
   this behavior. Changing the location of the signal trampoline
   makes it more difficult to reproduce the problem. I have not
   been because the able to completely mitigate it. When a cpu-thread
   stalls in this manner it appears to stall INSIDE the microcode
   for IRETQ. It doesn't make it to the return pc, and the cpu thread
   cannot take any IPIs or other hardware interrupts while in this
   state."
since the system instability has been observed on FreeBSD with SMT
disabled.  Interrupts to appear to play a factor since running a
signal-intensive process on the first CPU core, which handles most
of the interrupts on my machine, is far more likely to trigger the
problem than running such a process on any other core.

Also lower sv_maxuser to prevent a malicious user from using mmap()
to load and execute code in the top page of user memory that was made
available when the shared page was moved down.

Make the same changes to the 64-bit Linux emulator.

PR:		219399
Reported by:	nbe@renzel.net
Reviewed by:	kib
Reviewed by:	dchagin (previous version)
Tested by:	nbe@renzel.net (earlier version)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D11780

sys/amd64/amd64/elf_machdep.c

@@ -84,6 +84,25 @@ struct sysentvec elf64_freebsd_sysvec = {
 };
 INIT_SYSENTVEC(elf64_sysvec, &elf64_freebsd_sysvec);
 
+void
+amd64_lower_shared_page(struct sysentvec *sv)
+{
+	if (hw_lower_amd64_sharedpage != 0) {
+		sv->sv_maxuser -= PAGE_SIZE;
+		sv->sv_shared_page_base -= PAGE_SIZE;
+		sv->sv_usrstack -= PAGE_SIZE;
+		sv->sv_psstrings -= PAGE_SIZE;
+	}
+}
+
+/*
+ * Do this fixup before INIT_SYSENTVEC (SI_ORDER_ANY) because the latter
+ * uses the value of sv_shared_page_base.
+ */
+SYSINIT(elf64_sysvec_fixup, SI_SUB_EXEC, SI_ORDER_FIRST,
+	(sysinit_cfunc_t) amd64_lower_shared_page,
+	&elf64_freebsd_sysvec);
+
 static Elf64_Brandinfo freebsd_brand_info = {
 	.brand		= ELFOSABI_FREEBSD,
 	.machine	= EM_X86_64,

sys/amd64/amd64/initcpu.c

@@ -48,6 +48,11 @@ __FBSDID("$FreeBSD$");
 static int	hw_instruction_sse;
 SYSCTL_INT(_hw, OID_AUTO, instruction_sse, CTLFLAG_RD,
     &hw_instruction_sse, 0, "SIMD/MMX2 instructions available in CPU");
+static int	lower_sharedpage_init;
+int		hw_lower_amd64_sharedpage;
+SYSCTL_INT(_hw, OID_AUTO, lower_amd64_sharedpage, CTLFLAG_RDTUN,
+    &hw_lower_amd64_sharedpage, 0,
+   "Lower sharedpage to work around Ryzen issue with executing code near the top of user memory");
 /*
  * -1: automatic (default)
  *  0: keep enable CLFLUSH
@@ -122,6 +127,28 @@ init_amd(void)
 			wrmsr(0xc0011020, msr);
 		}
 	}
+
+	/*
+	 * Work around a problem on Ryzen that is triggered by executing
+	 * code near the top of user memory, in our case the signal
+	 * trampoline code in the shared page on amd64.
+	 *
+	 * This function is executed once for the BSP before tunables take
+	 * effect so the value determined here can be overridden by the
+	 * tunable.  This function is then executed again for each AP and
+	 * also on resume.  Set a flag the first time so that value set by
+	 * the tunable is not overwritten.
+	 *
+	 * The stepping and/or microcode versions should be checked after
+	 * this issue is fixed by AMD so that we don't use this mode if not
+	 * needed.
+	 */
+	if (lower_sharedpage_init == 0) {
+		lower_sharedpage_init = 1;
+		if (CPUID_TO_FAMILY(cpu_id) == 0x17) {
+			hw_lower_amd64_sharedpage = 1;
+		}
+	}
 }
 
 /*

sys/amd64/include/md_var.h

@@ -34,11 +34,14 @@
 
 #include <x86/x86_var.h>
 
-extern  uint64_t *vm_page_dump;
+extern uint64_t	*vm_page_dump;
+extern int	hw_lower_amd64_sharedpage;
 
 struct	savefpu;
+struct	sysentvec;
 
 void	amd64_db_resume_dbreg(void);
+void	amd64_lower_shared_page(struct sysentvec *);
 void	amd64_syscall(struct thread *td, int traced);
 void	doreti_iret(void) __asm(__STRING(doreti_iret));
 void	doreti_iret_fault(void) __asm(__STRING(doreti_iret_fault));

sys/amd64/linux/linux_sysvec.c

@@ -833,6 +833,8 @@ static void
 linux_vdso_install(void *param)
 {
 
+	amd64_lower_shared_page(&elf_linux_sysvec);
+
 	linux_szsigcode = (&_binary_linux_locore_o_end - 
 	    &_binary_linux_locore_o_start);