[Minios-devel] [UNIKRAFT PATCH 08/23] plat/xen/x86/entry64.S: Add more comments for critical region fixup

[Costin Lupu <costin.lupu@xxxxxxxxx>]

The implementation for handling event on Xen was ported from Mini-OS and has a
race condition after handling an event and before returning to user (more
details in entry64.S code). In order to support preemptive scheduling on Xen,
the fix for the race condition had to be properly understood and extended.
Therefore this patch adds the comments which helped me towards these goals.

Signed-off-by: Costin Lupu <costin.lupu@xxxxxxxxx>
---
 plat/xen/x86/entry64.S | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/plat/xen/x86/entry64.S b/plat/xen/x86/entry64.S
index d34181be..1aba52c1 100644
--- a/plat/xen/x86/entry64.S
+++ b/plat/xen/x86/entry64.S
@@ -274,21 +274,21 @@ hypervisor_callback2:
        cmpq $ecrit,%rax
        jb  critical_region_fixup
 
-11:    movq %gs:8,%rax
-       incl %gs:0
-       cmovzq %rax,%rsp
-       pushq %rdi
-       call do_hypervisor_callback
-       popq %rsp
-       decl %gs:0
+11:    movq %gs:8, %rax                # load IRQ stack
+       incl %gs:0                      # increment nested IRQ counter
+       cmovzq %rax, %rsp               # if not nested IRQ, then set IRQ sp
+       pushq %rdi                      # save last sp
+       call do_hypervisor_callback     # handle IRQ/event
+       popq %rsp                       # restore last sp
+       decl %gs:0                      # decrement nested IRQ counter
 
 error_exit:
        movl OFFSETOF_REGS_EFLAGS(%rsp), %eax
-       shr $9, %eax                    # EAX[0] == IRET_RFLAGS.IF
+       shr $9, %eax                    # EAX[0] == IRET_RFLAGS.IF
        XEN_GET_VCPU_INFO(%rsi)
-       andb evtchn_upcall_mask(%rsi),%al
-       andb $1,%al                     # EAX[0] == IRET_RFLAGS.IF & event_mask
-       jnz restore_all_enable_events   #        != 0 => enable event delivery
+       andb evtchn_upcall_mask(%rsi), %al
+       andb $1, %al                    # EAX[0] == IRET_RFLAGS.IF & event_mask
+       jnz restore_all_enable_events   #        != 0 => enable event delivery
 
        RESTORE_ALL
        HYPERVISOR_IRET
@@ -356,7 +356,7 @@ critical_region_fixup:
        andb $KERNEL_CS_MASK,OFFSETOF_REGS_CS(%rsp)      # CS might have changed
        jmp  11b
 
-#else
+#else /* !CONFIG_PARAVIRT */
 error_exit:
        RESTORE_ALL
        HYPERVISOR_IRET
@@ -365,7 +365,7 @@ error_exit:
  * Xen event (virtual interrupt) entry point.
  */
 TRAP_ENTRY hypervisor_callback, 0
-#endif
+#endif /* CONFIG_PARAVIRT */
 
 ENTRY(asm_failsafe_callback)
 #ifdef CONFIG_PARAVIRT
-- 
2.20.1


_______________________________________________
Minios-devel mailing list
Minios-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/minios-devel