[Xen-devel] [PATCH 1/2] x86/entry/64: Rearrange SYSCALL entry points for Xen's benefit

[Andy Lutomirski <luto@xxxxxxxxxx>]

Xen currently fudges RSP on SYSCALL to be compatible with the native
entries.  This has the unfortunate side effect that there are extra
poorly-controlled places with user RSP.  Add better entry points for
Xen to use instead.

This will add a couple of cycles of IRQ latency, but it avoids an
annoying corner case in which an IRQ can be delivered with a
hardware frame that overlaps current_pt_regs.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/entry_64.S        | 20 ++++++++++++++------
 arch/x86/entry/entry_64_compat.S | 10 ++++++----
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 041a37a643e1..f47996910331 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -148,17 +148,19 @@ GLOBAL(entry_SYSCALL_64_after_swapgs)
        /* Construct struct pt_regs on stack */
        pushq   $__USER_DS                      /* pt_regs->ss */
        pushq   PER_CPU_VAR(rsp_scratch)        /* pt_regs->sp */
+
        /*
-        * Re-enable interrupts.
-        * We use 'rsp_scratch' as a scratch space, hence irq-off block above
-        * must execute atomically in the face of possible interrupt-driven
-        * task preemption. We must enable interrupts only after we're done
-        * with using rsp_scratch:
+        * Before this point, interrupts MUST be off to prevent rsp_scratch
+        * from getting corrupted due to preemption.  Nonetheless, we keep
+        * interrupts off a little longer.  We eventually want to make it
+        * all the way to C code with interrupts off, which will considerably
+        * simplify context tracking.
         */
-       ENABLE_INTERRUPTS(CLBR_NONE)
+
        pushq   %r11                            /* pt_regs->flags */
        pushq   $__USER_CS                      /* pt_regs->cs */
        pushq   %rcx                            /* pt_regs->ip */
+GLOBAL(entry_SYSCALL_64_after_hwframe)
        pushq   %rax                            /* pt_regs->orig_ax */
        pushq   %rdi                            /* pt_regs->di */
        pushq   %rsi                            /* pt_regs->si */
@@ -171,6 +173,12 @@ GLOBAL(entry_SYSCALL_64_after_swapgs)
        pushq   %r11                            /* pt_regs->r11 */
        sub     $(6*8), %rsp                    /* pt_regs->bp, bx, r12-15 not 
saved */
 
+       /*
+        * Re-enable interrupts.  IRQ tracing already thinks they're
+        * on because user mode is traced as IRQs-on.
+        */
+       ENABLE_INTERRUPTS(CLBR_NONE)
+
        testl   $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, 
SIZEOF_PTREGS)
        jnz     tracesys
 entry_SYSCALL_64_fastpath:
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index d7571532e7ce..d39495d6446e 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -312,10 +312,6 @@ ENTRY(entry_SYSCALL_compat)
        SWAPGS_UNSAFE_STACK
        movl    %esp, %r8d
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
-       ENABLE_INTERRUPTS(CLBR_NONE)
-
-       /* Zero-extending 32-bit regs, do not remove */
-       movl    %eax, %eax
 
        /* Construct struct pt_regs on stack */
        pushq   $__USER32_DS            /* pt_regs->ss */
@@ -323,6 +319,7 @@ ENTRY(entry_SYSCALL_compat)
        pushq   %r11                    /* pt_regs->flags */
        pushq   $__USER32_CS            /* pt_regs->cs */
        pushq   %rcx                    /* pt_regs->ip */
+GLOBAL(entry_SYSCALL_compat_after_hwframe_esp_in_r8d)
        pushq   %rax                    /* pt_regs->orig_ax */
        pushq   %rdi                    /* pt_regs->di */
        pushq   %rsi                    /* pt_regs->si */
@@ -332,6 +329,11 @@ ENTRY(entry_SYSCALL_compat)
        pushq   $-ENOSYS                /* pt_regs->ax */
        sub     $(10*8), %rsp           /* pt_regs->r8-11, bp, bx, r12-15 not 
saved */
 
+       ENABLE_INTERRUPTS(CLBR_NONE)
+
+       /* Zero-extending 32-bit regs, do not remove */
+       movl    %eax, %eax
+
        /*
         * No need to do an access_ok check here because r8 has been
         * 32-bit zero extended:
-- 
2.4.3


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