| blob | 4f420c2f2d5534ea4ac5af20292e25c4346c3cda |
1 #include <linux/linkage.h>
2 #include <linux/lguest.h>
3 #include <asm/lguest_hcall.h>
4 #include <asm/asm-offsets.h>
5 #include <asm/thread_info.h>
6 #include <asm/processor-flags.h>
8 /*G:020
9 * Our story starts with the kernel booting into startup_32 in
10 * arch/x86/kernel/head_32.S. It expects a boot header, which is created by
11 * the bootloader (the Launcher in our case).
12 *
13 * The startup_32 function does very little: it clears the uninitialized global
14 * C variables which we expect to be zero (ie. BSS) and then copies the boot
15 * header and kernel command line somewhere safe. Finally it checks the
16 * 'hardware_subarch' field. This was introduced in 2.6.24 for lguest and Xen:
17 * if it's set to '1' (lguest's assigned number), then it calls us here.
18 *
19 * WARNING: be very careful here! We're running at addresses equal to physical
20 * addesses (around 0), not above PAGE_OFFSET as most code expectes
21 * (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any
22 * data without remembering to subtract __PAGE_OFFSET!
23 *
24 * The .section line puts this code in .init.text so it will be discarded after
25 * boot.
26 */
27 .section .init.text, "ax", @progbits
28 ENTRY(lguest_entry)
29 /*
30 * We make the "initialization" hypercall now to tell the Host about
31 * us, and also find out where it put our page tables.
32 */
33 movl $LHCALL_LGUEST_INIT, %eax
34 movl $lguest_data - __PAGE_OFFSET, %ebx
35 int $LGUEST_TRAP_ENTRY
37 /* Set up the initial stack so we can run C code. */
38 movl $(init_thread_union+THREAD_SIZE),%esp
40 /* Jumps are relative: we're running __PAGE_OFFSET too low. */
41 jmp lguest_init+__PAGE_OFFSET
43 /*G:055
44 * We create a macro which puts the assembler code between lgstart_ and lgend_
45 * markers. These templates are put in the .text section: they can't be
46 * discarded after boot as we may need to patch modules, too.
47 */
48 .text
49 #define LGUEST_PATCH(name, insns...) \
50 lgstart_##name: insns; lgend_##name:; \
51 .globl lgstart_##name; .globl lgend_##name
53 LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
54 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
56 /*G:033
57 * But using those wrappers is inefficient (we'll see why that doesn't matter
58 * for save_fl and irq_disable later). If we write our routines carefully in
59 * assembler, we can avoid clobbering any registers and avoid jumping through
60 * the wrapper functions.
61 *
62 * I skipped over our first piece of assembler, but this one is worth studying
63 * in a bit more detail so I'll describe in easy stages. First, the routine to
64 * enable interrupts:
65 */
66 ENTRY(lg_irq_enable)
67 /*
68 * The reverse of irq_disable, this sets lguest_data.irq_enabled to
69 * X86_EFLAGS_IF (ie. "Interrupts enabled").
70 */
71 movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
72 /*
73 * But now we need to check if the Host wants to know: there might have
74 * been interrupts waiting to be delivered, in which case it will have
75 * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we
76 * jump to send_interrupts, otherwise we're done.
77 */
78 testl $0, lguest_data+LGUEST_DATA_irq_pending
79 jnz send_interrupts
80 /*
81 * One cool thing about x86 is that you can do many things without using
82 * a register. In this case, the normal path hasn't needed to save or
83 * restore any registers at all!
84 */
85 ret
86 send_interrupts:
87 /*
88 * OK, now we need a register: eax is used for the hypercall number,
89 * which is LHCALL_SEND_INTERRUPTS.
90 *
91 * We used not to bother with this pending detection at all, which was
92 * much simpler. Sooner or later the Host would realize it had to
93 * send us an interrupt. But that turns out to make performance 7
94 * times worse on a simple tcp benchmark. So now we do this the hard
95 * way.
96 */
97 pushl %eax
98 movl $LHCALL_SEND_INTERRUPTS, %eax
99 /*
100 * This is a vmcall instruction (same thing that KVM uses). Older
101 * assembler versions might not know the "vmcall" instruction, so we
102 * create one manually here.
103 */
104 .byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
105 /* Put eax back the way we found it. */
106 popl %eax
107 ret
109 /*
110 * Finally, the "popf" or "restore flags" routine. The %eax register holds the
111 * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're
112 * enabling interrupts again, if it's 0 we're leaving them off.
113 */
114 ENTRY(lg_restore_fl)
115 /* This is just "lguest_data.irq_enabled = flags;" */
116 movl %eax, lguest_data+LGUEST_DATA_irq_enabled
117 /*
118 * Now, if the %eax value has enabled interrupts and
119 * lguest_data.irq_pending is set, we want to tell the Host so it can
120 * deliver any outstanding interrupts. Fortunately, both values will
121 * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl"
122 * instruction will AND them together for us. If both are set, we
123 * jump to send_interrupts.
124 */
125 testl lguest_data+LGUEST_DATA_irq_pending, %eax
126 jnz send_interrupts
127 /* Again, the normal path has used no extra registers. Clever, huh? */
128 ret
129 /*:*/
131 /* These demark the EIP range where host should never deliver interrupts. */
132 .global lguest_noirq_start
133 .global lguest_noirq_end
135 /*M:004
136 * When the Host reflects a trap or injects an interrupt into the Guest, it
137 * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled,
138 * so the Guest iret logic does the right thing when restoring it. However,
139 * when the Host sets the Guest up for direct traps, such as system calls, the
140 * processor is the one to push eflags onto the stack, and the interrupt bit
141 * will be 1 (in reality, interrupts are always enabled in the Guest).
142 *
143 * This turns out to be harmless: the only trap which should happen under Linux
144 * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
145 * regions), which has to be reflected through the Host anyway. If another
146 * trap *does* go off when interrupts are disabled, the Guest will panic, and
147 * we'll never get to this iret!
148 :*/
150 /*G:045
151 * There is one final paravirt_op that the Guest implements, and glancing at it
152 * you can see why I left it to last. It's *cool*! It's in *assembler*!
153 *
154 * The "iret" instruction is used to return from an interrupt or trap. The
155 * stack looks like this:
156 * old address
157 * old code segment & privilege level
158 * old processor flags ("eflags")
159 *
160 * The "iret" instruction pops those values off the stack and restores them all
161 * at once. The only problem is that eflags includes the Interrupt Flag which
162 * the Guest can't change: the CPU will simply ignore it when we do an "iret".
163 * So we have to copy eflags from the stack to lguest_data.irq_enabled before
164 * we do the "iret".
165 *
166 * There are two problems with this: firstly, we need to use a register to do
167 * the copy and secondly, the whole thing needs to be atomic. The first
168 * problem is easy to solve: push %eax on the stack so we can use it, and then
169 * restore it at the end just before the real "iret".
170 *
171 * The second is harder: copying eflags to lguest_data.irq_enabled will turn
172 * interrupts on before we're finished, so we could be interrupted before we
173 * return to userspace or wherever. Our solution to this is to surround the
174 * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the
175 * Host that it is *never* to interrupt us there, even if interrupts seem to be
176 * enabled.
177 */
178 ENTRY(lguest_iret)
179 pushl %eax
180 movl 12(%esp), %eax
181 lguest_noirq_start:
182 /*
183 * Note the %ss: segment prefix here. Normal data accesses use the
184 * "ds" segment, but that will have already been restored for whatever
185 * we're returning to (such as userspace): we can't trust it. The %ss:
186 * prefix makes sure we use the stack segment, which is still valid.
187 */
188 movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
189 popl %eax
190 iret
191 lguest_noirq_end: