We need this since glibc uses ASM that uses the red-zone. Simply compiling
with -mno-red-zone is insufficient.
"And tell me, Mr. Anderson, what good is a compiler flag if you are
unable to compile?"
The plus side is we can use glibc's ASM and we can get slightly better perf
by using the red-zone. The downside is that every attempt to pop a HW TF
burns 128 bytes more of the uthread's stack. That decreases the number of
contiguous failures we can handle before running of the end of the user's
stack. It's a good thing we just did commit 7506964a4e71 ("Check
notif_pending early in pop_user_ctx()"), which decreases the chance of
those errors. We'll see...
Signed-off-by: Barret Rhoden <[email protected]>
---
user/parlib/x86/vcore.c | 37 ++++++++++++++++++++-----------------
1 file changed, 20 insertions(+), 17 deletions(-)
diff --git a/user/parlib/x86/vcore.c b/user/parlib/x86/vcore.c
index ea38099401c0..3f7b6c862b15 100644
--- a/user/parlib/x86/vcore.c
+++ b/user/parlib/x86/vcore.c
@@ -10,10 +10,20 @@
* enable notifs, make sure notif IPIs weren't pending, restore the work reg,
* and then "ret".
*
+ * However, we can't just put the extra stuff directly below the rsp. We need
+ * to leave room for the redzone: area that is potentially being used. (Even
if
+ * you compile with -mno-red-zone, some asm code (glibc memcpy) will still use
+ * that area).
+ *
* This is what the target uthread's stack will look like (growing down):
*
* Target RSP -> | u_thread's old stuff | the future %rsp, tf->tf_rsp
- * | new rip | 0x08 below %rsp (one slot is
0x08)
+ * | . | beginning of Red Zone
+ * | . |
+ * | 128 Bytes of Red Zone |
+ * | . |
+ * | . | end of Red Zone
+ * | new rip | 0x08 below Red (one slot is 0x08)
* | rflags space | 0x10 below
* | rdi save space | 0x18 below
* | *sysc ptr to syscall | 0x20 below
@@ -29,18 +39,7 @@
* so that restarting *this* code won't clobber shit we need later. The way we
* do this is that we do any "stack jumping" before we enable
interrupts/notifs.
* These jumps are when we directly modify rsp, specifically in the down
- * direction (subtracts). Adds would be okay.
- *
- * Another 64-bit concern is the red-zone. The AMD64 ABI allows the use of
- * space below the stack pointer by regular programs. If we allowed this, we
- * would clobber that space when we do our TF restarts, much like with OSs and
- * IRQ handlers. Thus we have the cross compiler automatically disabling the
- * redzone (-mno-red-zone is a built-in option).
- *
- * When compared to the 32 bit code, notice we use rdi, instead of eax, for our
- * work. This is because rdi is the arg0 of a syscall. Using it saves us some
- * extra moves, since we need to pop the *sysc before saving any other
- * registers. */
+ * direction (subtracts). Adds would be okay. */
/* Helper for writing the info we need later to the u_tf's stack. Also, note
* this goes backwards, since memory reads up the stack. */
@@ -52,6 +51,7 @@ struct restart_helper {
uint64_t rflags;
uint64_t rip;
};
+
/* Static syscall, used for self-notifying. We never wait on it, and we
* actually might submit it multiple times in parallel on different cores!
* While this may seem dangerous, the kernel needs to be able to handle this
@@ -74,11 +74,13 @@ struct syscall vc_entry = {
static void pop_hw_tf(struct hw_trapframe *tf, uint32_t vcoreid)
{
+ #define X86_RED_ZONE_SZ 128
struct restart_helper *rst;
struct preempt_data *vcpd = &__procdata.vcore_preempt_data[vcoreid];
- /* The stuff we need to write will be below the current stack of the
utf */
- rst = (struct restart_helper*)((void*)tf->tf_rsp -
+ /* The stuff we need to write will be below the current stack and red
zone
+ * of the utf */
+ rst = (struct restart_helper*)((void*)tf->tf_rsp - X86_RED_ZONE_SZ -
sizeof(struct restart_helper));
/* Fill in the info we'll need later */
rst->notif_disab_loc = &vcpd->notif_disabled;
@@ -106,6 +108,7 @@ static void pop_hw_tf(struct hw_trapframe *tf, uint32_t
vcoreid)
"popq %%r15; "
"addq $0x28, %%rsp; " /* move to the rsp slot in the
tf */
"popq %%rsp; " /* change to the utf's %rsp */
+ "subq %[red], %%rsp; " /* jump over the redzone */
"subq $0x10, %%rsp; " /* move rsp to below rdi's slot
*/
"pushq %%rdi; " /* save rdi, will clobber soon */
"subq $0x18, %%rsp; " /* move to notif_dis_loc slot */
@@ -131,9 +134,9 @@ static void pop_hw_tf(struct hw_trapframe *tf, uint32_t
vcoreid)
"1: addq $0x08, %%rsp; " /* discard &sysc (on
non-sc path) */
"2: popq %%rdi; " /* restore tf's %rdi (both
paths) */
"popfq; " /* restore utf's
rflags */
- "ret; " /* return to the new PC */
+ "ret %[red]; " /* return to the new PC, skip
red */
:
- : "g"(&tf->tf_rax), "i"(T_SYSCALL)
+ : "g"(&tf->tf_rax), "i"(T_SYSCALL),
[red]"i"(X86_RED_ZONE_SZ)
: "memory");
}
--
2.8.0.rc3.226.g39d4020
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