1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
|
/*
* Asm versions of Xen pv-ops, suitable for either direct use or
* inlining. The inline versions are the same as the direct-use
* versions, with the pre- and post-amble chopped off.
*
* This code is encoded for size rather than absolute efficiency, with
* a view to being able to inline as much as possible.
*
* We only bother with direct forms (ie, vcpu in pda) of the
* operations here; the indirect forms are better handled in C, since
* they're generally too large to inline anyway.
*/
#include <asm/thread_info.h>
#include <asm/processor-flags.h>
#include <asm/segment.h>
#include <asm/asm.h>
#include <xen/interface/xen.h>
#include "xen-asm.h"
/*
* Force an event check by making a hypercall, but preserve regs
* before making the call.
*/
check_events:
push %eax
push %ecx
push %edx
call xen_force_evtchn_callback
pop %edx
pop %ecx
pop %eax
ret
/*
* 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. If the nested
* interrupt ends up setting one of the TIF_WORK_MASK pending work
* flags, they will not be tested again before returning to
* usermode. This means that a process can end up with pending work,
* which will be unprocessed until the process enters and leaves the
* kernel again, which could be an unbounded amount of time. This
* means that a pending signal or reschedule event could be
* indefinitely delayed.
*
* The fix is to notice a nested interrupt in the critical window, and
* if one occurs, then fold the nested interrupt into the current
* interrupt stack frame, and re-process it iteratively rather than
* recursively. This means that it will exit via the normal path, and
* all pending work will be dealt with appropriately.
*
* Because the nested interrupt handler needs to deal with the current
* stack state in whatever form its in, we keep things simple by only
* using a single register which is pushed/popped on the stack.
*/
.macro POP_FS
1:
popw %fs
.pushsection .fixup, "ax"
2: movw $0, (%esp)
jmp 1b
.popsection
_ASM_EXTABLE(1b,2b)
.endm
ENTRY(xen_iret)
/* test eflags for special cases */
testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
jnz hyper_iret
push %eax
ESP_OFFSET=4 # bytes pushed onto stack
/* Store vcpu_info pointer for easy access */
#ifdef CONFIG_SMP
pushw %fs
movl $(__KERNEL_PERCPU), %eax
movl %eax, %fs
movl %fs:xen_vcpu, %eax
POP_FS
#else
movl %ss:xen_vcpu, %eax
#endif
/* check IF state we're restoring */
testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)
/*
* 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 %ss:XEN_vcpu_info_mask(%eax)
xen_iret_start_crit:
/* check for unmasked and pending */
cmpw $0x0001, %ss:XEN_vcpu_info_pending(%eax)
/*
* If there's something pending, mask events again so we can
* jump back into xen_hypervisor_callback. Otherwise do not
* touch XEN_vcpu_info_mask.
*/
jne 1f
movb $1, %ss:XEN_vcpu_info_mask(%eax)
1: 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
1: iret
xen_iret_end_crit:
_ASM_EXTABLE(1b, iret_exc)
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 : (ss/esp may be present if we came from usermode)
* 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 }<- 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 it's usermode state which we
* eventually need to restore.
*/
ENTRY(xen_iret_crit_fixup)
/*
* Paranoia: Make sure we're really coming from kernel space.
* One 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. The Intel documents
* explicitly say that the reported EIP for a bad jump is the
* jump instruction itself, not the destination, but some
* virtual environments get this wrong.
*/
movl PT_CS(%esp), %ecx
andl $SEGMENT_RPL_MASK, %ecx
cmpl $USER_RPL, %ecx
je 2f
lea PT_ORIG_EAX(%esp), %esi
lea PT_EFLAGS(%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 (just above top of frame) */
movl %eax, PT_EAX(%esp)
lea ESP_OFFSET(%edi), %edi /* move dest up over saved regs */
/* set up the copy */
1: std
mov $PT_EIP / 4, %ecx /* saved regs up to orig_eax */
rep movsl
cld
lea 4(%edi), %esp /* point esp to new frame */
2: jmp xen_do_upcall
|
