[PATCH] xen: use iret directly where possible

[Jeremy Fitzhardinge <jeremy@goop.org>]

xen: use iret directly where possible

Most of the time we can simply use the iret instruction to exit the
kernel, rather than having to use the iret hypercall - the only
exception is if we're returning into vm86 mode, or from delivering an
NMI (which we don't support yet).

When running native, iret has the behaviour of testing for a pending
interrupt atomically with re-enabling interrupts.  Unfortunately
there's no way to do this with Xen, so there's a window in which we
could get a recursive exception after enabling events but before
actually returning to userspace.

This causes three problems:

 - If the interrupt causes a softirq to be queued, we will return to
   userspace without processing it, since its already after the point at
   which we look for queued softirqs.  This means it could be an
   unbounded amount of time before it gets processed on next kernel
   entry.

 - If the interrupt causes a signal to be delivered to the current process,
   the signal will be marked pending on the process, but it will not
   get delivered because we're past the point where pending signals
   are detected.  Again, it could be an unbounded amount of time
   before the signal gets delivered.

 - The recursion is, in theory, unbounded.  There's a small chance that
   a series of unfortunate events will cause the exception frames to
   build up and overrun the stack.  But that's very unlikely.

To deal with this, the xen event upcall handler checks to see if the
EIP is within the critical section of the iret code, after events
are (potentially) enabled up to the iret itself.  If its within this
range, it calls the iret critical section fixup, which adjusts the
stack to deal with any unrestored registers, and then shifts the
stack frame up to replace the previous invocation.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>

---
 arch/i386/kernel/asm-offsets.c |    1 
 arch/i386/kernel/entry.S       |   16 +++
 arch/i386/xen/enlighten.c      |    1 
 arch/i386/xen/xen-asm.S        |  167 +++++++++++++++++++++++++++++++++++++++-
 arch/i386/xen/xen-ops.h        |    1 
 5 files changed, 181 insertions(+), 5 deletions(-)

===================================================================
--- a/arch/i386/kernel/asm-offsets.c
+++ b/arch/i386/kernel/asm-offsets.c
@@ -65,6 +65,7 @@ void foo(void)
 	OFFSET(TI_addr_limit, thread_info, addr_limit);
 	OFFSET(TI_restart_block, thread_info, restart_block);
 	OFFSET(TI_sysenter_return, thread_info, sysenter_return);
+	OFFSET(TI_cpu, thread_info, cpu);
 	BLANK();
 
 	OFFSET(GDS_size, Xgt_desc_struct, size);
===================================================================
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -1030,7 +1030,21 @@ ENTRY(xen_hypervisor_callback)
 	CFI_ADJUST_CFA_OFFSET 4
 	SAVE_ALL
 	TRACE_IRQS_OFF
-	mov %esp, %eax
+
+	/* Check to see if we got the event in the critical
+	   region in xen_iret_direct, after we've reenabled
+	   events and checked for pending events.  This simulates
+	   iret instruction's behaviour where it delivers a
+	   pending interrupt when enabling interrupts. */
+	movl PT_EIP(%esp),%eax
+	cmpl $xen_iret_start_crit,%eax
+	jb   1f
+	cmpl $xen_iret_end_crit,%eax
+	jae  1f
+
+	call xen_iret_crit_fixup
+
+1:	mov %esp, %eax
 	call xen_evtchn_do_upcall
 	jmp  ret_from_intr
 	CFI_ENDPROC
===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -827,6 +827,7 @@ void __init xen_setup_vcpu_info_placemen
 		paravirt_ops.irq_disable = xen_irq_disable_direct;
 		paravirt_ops.irq_enable = xen_irq_enable_direct;
 		paravirt_ops.read_cr2 = xen_read_cr2_direct;
+		paravirt_ops.iret = xen_iret_direct;
 	}
 }
 
===================================================================
--- a/arch/i386/xen/xen-asm.S
+++ b/arch/i386/xen/xen-asm.S
@@ -12,14 +12,20 @@
  */
 
 #include <linux/linkage.h>
+
 #include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
 #include <asm/percpu.h>
-#include <asm/asm-offsets.h>
 #include <asm/processor-flags.h>
+#include <asm/segment.h>
+
+#include <xen/interface/xen.h>
 
 #define RELOC(x, v)	.globl x##_reloc; x##_reloc=v
 #define ENDPATCH(x)	.globl x##_end; x##_end=.
+
+/* Pseudo-flag used for virtual NMI, which we don't implement yet */
+#define XEN_EFLAGS_NMI	0x80000000
 
 /*
 	Enable events.  This clears the event mask and tests the pending
@@ -81,13 +87,12 @@ ENDPATCH(xen_save_fl_direct)
  */
 ENTRY(xen_restore_fl_direct)
 	testb $X86_EFLAGS_IF>>8, %ah
-	setz %al
-	movb %al, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
+	setz PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
 	/* Preempt here doesn't matter because that will deal with
 	   any pending interrupts.  The pending check may end up being
 	   run on the wrong CPU, but that doesn't hurt. */
 
-	/* check for pending but unmasked */
+	/* check for unmasked and pending */
 	cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
 	jz 1f
 2:	call check_events
@@ -97,6 +102,160 @@ ENDPATCH(xen_restore_fl_direct)
 	ENDPROC(xen_restore_fl_direct)
 	RELOC(xen_restore_fl_direct, 2b+1)
 
+/*
+	This is run where a normal iret would be run, with the same stack setup:
+	      8: eflags
+	      4: cs
+	esp-> 0: eip
+
+	This attempts to make sure that any pending events are dealt with
+	on return to usermode, but there is a small window in which an event
+	can happen just before entering usermode.  This has three effects:
+	 - There can be interrupt recursion on the stack, which is
+	   unbounded in theory (but very unlikely in practice)
+	 - New softirq events can be queued up, but they won't get
+	   processed until the cpu next enters and leaves the kernel.
+	 - Signals likewise.
+
+	Non-direct iret could be done in the same way, but it would
+	require an annoying amount of code duplication.  We'll assume
+	that direct mode will be the common case once the hypervisor
+	support becomes commonplace.
+ */
+ENTRY(xen_iret_direct)
+	/* test eflags for special cases */
+	testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
+	jnz hyper_iret
+
+	/* check IF state we're restoring */
+	testb $X86_EFLAGS_IF>>8, 8+1(%esp)
+
+	push %eax
+	ESP_OFFSET=4	# bytes pushed onto stack
+
+	/* Store vcpu_info pointer for easy access.  Do it this
+	   way to avoid having to reload %fs */
+#ifdef CONFIG_SMP
+	GET_THREAD_INFO(%eax)
+	movl TI_cpu(%eax),%eax
+	movl __per_cpu_offset(,%eax,4),%eax
+	lea per_cpu__xen_vcpu_info(%eax),%eax
+#else
+	movl $per_cpu__xen_vcpu_info, %eax
+#endif
+
+	/* Maybe enable events.  Once this happens we could get a
+	   recursive event, so the critical region starts immediately
+	   afterwards.  However, if that happens we don't end up
+	   resuming the code, so we don't have to be worried about
+	   being preempted to another CPU. */
+	setz XEN_vcpu_info_mask(%eax)
+xen_iret_start_crit:
+
+	/* check for unmasked and pending */
+	cmpw $0x0001, XEN_vcpu_info_pending(%eax)
+
+	/* If there's something pending, mask events again so we
+	   can jump back into xen_hypervisor_callback */
+	sete XEN_vcpu_info_mask(%eax)
+
+	popl %eax
+
+	/* From this point on the registers are restored and the stack
+	   updated, so we don't need to worry about it if we're preempted */
+iret_restore_end:
+
+	/* Jump to hypervisor_callback after fixing up the stack.
+	   Events are masked, so jumping out of the critical
+	   region is OK. */
+	je xen_hypervisor_callback
+
+	iret
+xen_iret_end_crit:
+
+hyper_iret:
+	/* put this out of line since its very rarely used */
+	jmp hypercall_page + __HYPERVISOR_iret * 32
+
+	.globl xen_iret_start_crit, xen_iret_end_crit
+
+/*
+   This is called by xen_hypervisor_callback in entry.S when it sees
+   that the EIP at the time of interrupt was between xen_iret_start_crit
+   and xen_iret_end_crit.  We're passed the EIP in %eax so we can do
+   a more refined determination of what to do.
+
+   The stack format at this point is:
+	----------------
+	 ss		:
+	 esp		:
+	 eflags		}  outer exception info
+	 cs		}
+	 eip		}
+	---------------- <- edi (copy dest)
+	 eax		:  outer eax if it hasn't been restored
+	----------------
+	 eflags		}  nested exception info
+	 cs		}   (no ss/esp because we're nested
+	 eip		}    from the same ring)
+	 orig_eax	}<- esi (copy src)
+	 - - - - - - - -
+	 fs		}
+	 es		}
+	 ds		}  SAVE_ALL state
+	 eax		}
+	  :		:
+	 ebx		}
+	----------------
+	 return addr	 <- esp
+	----------------
+
+   In order to deliver the nested exception properly, we need to shift
+   everything from the return addr up to the error code so it
+   sits just under the outer exception info.  This means that when we
+   handle the exception, we do it in the context of the outer exception
+   rather than starting a new one.
+
+   The only caveat is that if the outer eax hasn't been
+   restored yet (ie, it's still on stack), we need to insert
+   its value into the SAVE_ALL state before going on, since
+   its usermode state which we eventually need to restore.
+ */
+ENTRY(xen_iret_crit_fixup)
+	/* offsets +4 for return address */
+
+	/* Paranoia: make sure we're really coming from userspace.
+	   Once could imagine a case where userspace jumps into
+	   the critical range address, but just before the CPU
+	   delivers a GP, it decides to deliver an interrupt
+	   instead.  Unlikely?  Definitely.  Easy to avoid?
+	   Yes. (Some virtual environments get this wrong.) */
+	movl PT_CS+4(%esp), %ecx
+	andl $SEGMENT_RPL_MASK, %ecx
+	cmpl $USER_RPL, %ecx
+	je 2f
+
+	lea PT_ORIG_EAX+4(%esp), %esi
+	lea PT_EFLAGS+4(%esp), %edi
+
+	/* If eip is before iret_restore_end then stack
+	   hasn't been restored yet. */
+	cmp $iret_restore_end, %eax
+	jae 1f
+
+	movl 0+4(%edi),%eax		/* copy EAX */
+	movl %eax, PT_EAX+4(%esp)
+
+	lea ESP_OFFSET(%edi),%edi	/* move dest up over saved regs */
+
+	/* set up the copy */
+1:	std
+	mov $(PT_EIP+4) / 4, %ecx	/* copy ret+saved regs up to orig_eax */
+	rep movsl
+	cld
+
+	lea 4(%edi),%esp		/* point esp to new frame */
+2:	ret
 
 
 /*
===================================================================
--- a/arch/i386/xen/xen-ops.h
+++ b/arch/i386/xen/xen-ops.h
@@ -68,4 +68,5 @@ DECL_ASM(unsigned long, xen_save_fl_dire
 DECL_ASM(unsigned long, xen_save_fl_direct, void);
 DECL_ASM(void, xen_restore_fl_direct, unsigned long);
 
+void xen_iret_direct(void);
 #endif /* XEN_OPS_H */

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