[Xen-devel] [PATCH V2 6/6] mini-os/x86-64 entry: check against nested events and try to fix up

[Xu Zhang <xzhang@xxxxxxxxxx>]

In hypervisor_callback, check against event re-entrant.
If we came from the critical region in interrupt context,
try to fix up by coalescing the two stack frames.
The execution is resumed as if the second event never happened.

Signed-off-by: Xu Zhang <xzhang@xxxxxxxxxx>
---
 extras/mini-os/arch/x86/x86_64.S |   87 +++++++++++++++++++++++++++++++------
 1 files changed, 73 insertions(+), 14 deletions(-)

diff --git a/extras/mini-os/arch/x86/x86_64.S b/extras/mini-os/arch/x86/x86_64.S
index 20ab477..f022eb3 100644
--- a/extras/mini-os/arch/x86/x86_64.S
+++ b/extras/mini-os/arch/x86/x86_64.S
@@ -57,10 +57,15 @@ hypercall_page:
 #define evtchn_upcall_mask             1
 
 NMI_MASK = 0x80000000
+KERNEL_CS_MASK = 0xfc
 
-#define RDI 112
-#define ORIG_RAX 120       /* + error_code */ 
-#define RFLAGS 144
+#define RAX       80
+#define RDI      112
+#define ORIG_RAX 120       /* + error_code */
+#define RIP      128
+#define CS       136
+#define RFLAGS   144
+#define RSP      152
 
 
 /* Macros */
@@ -176,6 +181,14 @@ ENTRY(hypervisor_callback)
 
 ENTRY(hypervisor_callback2)
        movq %rdi, %rsp
+
+       /* check against event re-entrant */
+       movq RIP(%rsp),%rax
+       cmpq $scrit,%rax
+       jb 11f
+       cmpq $ecrit,%rax
+       jb  critical_region_fixup
+
 11:    movq %gs:8,%rax
        incl %gs:0
        cmovzq %rax,%rsp
@@ -200,22 +213,68 @@ retint_restore_args:
        HYPERVISOR_IRET 0
 
 restore_all_enable_events:
-       XEN_UNBLOCK_EVENTS(%rsi)        # %rsi is already set up...
-
-scrit: /**** START OF CRITICAL REGION ****/
-       XEN_TEST_PENDING(%rsi)
-       jnz  14f                        # process more events if necessary...
-       XEN_PUT_VCPU_INFO(%rsi)
-
        RESTORE_REST
        RESTORE_ALL
+       pushq %rax                      # save rax for it will be clobbered 
later
+       RSP_OFFSET=8                    # record the stack frame layout changes
+       XEN_GET_VCPU_INFO(%rax)         # safe to use rax since it is saved
+       XEN_UNBLOCK_EVENTS(%rax)
+
+scrit: /**** START OF CRITICAL REGION ****/
+       XEN_TEST_PENDING(%rax)
+       jz 12f
+       XEN_LOCKED_BLOCK_EVENTS(%rax)   # if pending, mask events and handle
+                                       # by jumping to hypervisor_prologue
+12:    popq %rax                       # all registers restored from this point
+
+restore_end:
+       jnz hypervisor_prologue         # safe to jump out of critical region
+                                       # because events are masked if ZF = 0
        HYPERVISOR_IRET 0
+ecrit:  /**** END OF CRITICAL REGION ****/
 
-14:    XEN_LOCKED_BLOCK_EVENTS(%rsi)
-       XEN_PUT_VCPU_INFO(%rsi)
-       movq %rsp,%rdi                  # set the argument again
+# Set up the stack as Xen does before calling event callback
+hypervisor_prologue:
+       pushq %r11
+       pushq %rcx
+       jmp hypervisor_callback
+
+# [How we do the fixup]. We want to merge the current stack frame with the
+# just-interrupted frame. How we do this depends on where in the critical
+# region the interrupted handler was executing, and so if rax has been
+# restored. We determine by comparing interrupted rip with "restore_end".
+# We always copy all registers below RIP from the current stack frame
+# to the end of the previous activation frame so that we can continue
+# as if we've never even reached 11 running in the old activation frame.
+
+critical_region_fixup:
+       # Set up source and destination region pointers
+       leaq RIP(%rsp),%rsi   # esi points at end of src region
+       # Acquire interrupted rsp which was saved-on-stack. This points to
+       # the end of dst region. Note that it is not necessarily current rsp
+       # plus 0xb0, because the second interrupt might align the stack frame.
+       movq RSP(%rsp),%rdi   # edi points at end of dst region
+
+       cmpq $restore_end,%rax
+       jae  13f
+
+       # If interrupted rip is before restore_end
+       # then rax hasn't been restored yet
+       movq (%rdi),%rax
+       movq %rax, RAX(%rsp)  # save rax
+       addq $RSP_OFFSET,%rdi
+
+       # Set up the copy
+13:    movq $RIP,%rcx
+       shr  $3,%rcx          # convert bytes into count of 64-bit entities
+15:    subq $8,%rsi          # pre-decrementing copy loop
+       subq $8,%rdi
+       movq (%rsi),%rax
+       movq %rax,(%rdi)
+       loop 15b
+16:    movq %rdi,%rsp        # final rdi is top of merged stack
+       andb $KERNEL_CS_MASK,CS(%rsp)      # CS might have changed
        jmp  11b
-ecrit:  /**** END OF CRITICAL REGION ****/
 
 
 
-- 
1.7.7.6


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