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>
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).
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.
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!
24 * The .section line puts this code in .init.text so it will be discarded after
27 .section .init.text, "ax", @progbits
30 * We make the "initialization" hypercall now to tell the Host where
31 * our lguest_data struct is.
33 movl $LHCALL_LGUEST_INIT, %eax
34 movl $lguest_data - __PAGE_OFFSET, %ebx
35 int $LGUEST_TRAP_ENTRY
37 /* Now turn our pagetables on; setup by arch/x86/kernel/head_32.S. */
38 movl $LHCALL_NEW_PGTABLE, %eax
39 movl $(initial_page_table - __PAGE_OFFSET), %ebx
40 int $LGUEST_TRAP_ENTRY
42 /* Set up the initial stack so we can run C code. */
43 movl $(init_thread_union+THREAD_SIZE),%esp
45 /* Jumps are relative: we're running __PAGE_OFFSET too low. */
46 jmp lguest_init+__PAGE_OFFSET
49 * We create a macro which puts the assembler code between lgstart_ and lgend_
50 * markers. These templates are put in the .text section: they can't be
51 * discarded after boot as we may need to patch modules, too.
54 #define LGUEST_PATCH(name, insns...) \
55 lgstart_##name: insns; lgend_##name:; \
56 .globl lgstart_##name; .globl lgend_##name
58 LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
59 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
62 * But using those wrappers is inefficient (we'll see why that doesn't matter
63 * for save_fl and irq_disable later). If we write our routines carefully in
64 * assembler, we can avoid clobbering any registers and avoid jumping through
65 * the wrapper functions.
67 * I skipped over our first piece of assembler, but this one is worth studying
68 * in a bit more detail so I'll describe in easy stages. First, the routine to
73 * The reverse of irq_disable, this sets lguest_data.irq_enabled to
74 * X86_EFLAGS_IF (ie. "Interrupts enabled").
76 movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
78 * But now we need to check if the Host wants to know: there might have
79 * been interrupts waiting to be delivered, in which case it will have
80 * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we
81 * jump to send_interrupts, otherwise we're done.
83 testl $0, lguest_data+LGUEST_DATA_irq_pending
86 * One cool thing about x86 is that you can do many things without using
87 * a register. In this case, the normal path hasn't needed to save or
88 * restore any registers at all!
93 * OK, now we need a register: eax is used for the hypercall number,
94 * which is LHCALL_SEND_INTERRUPTS.
96 * We used not to bother with this pending detection at all, which was
97 * much simpler. Sooner or later the Host would realize it had to
98 * send us an interrupt. But that turns out to make performance 7
99 * times worse on a simple tcp benchmark. So now we do this the hard
103 movl $LHCALL_SEND_INTERRUPTS, %eax
105 * This is a vmcall instruction (same thing that KVM uses). Older
106 * assembler versions might not know the "vmcall" instruction, so we
107 * create one manually here.
109 .byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
110 /* Put eax back the way we found it. */
115 * Finally, the "popf" or "restore flags" routine. The %eax register holds the
116 * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're
117 * enabling interrupts again, if it's 0 we're leaving them off.
120 /* This is just "lguest_data.irq_enabled = flags;" */
121 movl %eax, lguest_data+LGUEST_DATA_irq_enabled
123 * Now, if the %eax value has enabled interrupts and
124 * lguest_data.irq_pending is set, we want to tell the Host so it can
125 * deliver any outstanding interrupts. Fortunately, both values will
126 * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl"
127 * instruction will AND them together for us. If both are set, we
128 * jump to send_interrupts.
130 testl lguest_data+LGUEST_DATA_irq_pending, %eax
132 /* Again, the normal path has used no extra registers. Clever, huh? */
136 /* These demark the EIP range where host should never deliver interrupts. */
137 .global lguest_noirq_start
138 .global lguest_noirq_end
141 * When the Host reflects a trap or injects an interrupt into the Guest, it
142 * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled,
143 * so the Guest iret logic does the right thing when restoring it. However,
144 * when the Host sets the Guest up for direct traps, such as system calls, the
145 * processor is the one to push eflags onto the stack, and the interrupt bit
146 * will be 1 (in reality, interrupts are always enabled in the Guest).
148 * This turns out to be harmless: the only trap which should happen under Linux
149 * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
150 * regions), which has to be reflected through the Host anyway. If another
151 * trap *does* go off when interrupts are disabled, the Guest will panic, and
152 * we'll never get to this iret!
156 * There is one final paravirt_op that the Guest implements, and glancing at it
157 * you can see why I left it to last. It's *cool*! It's in *assembler*!
159 * The "iret" instruction is used to return from an interrupt or trap. The
160 * stack looks like this:
162 * old code segment & privilege level
163 * old processor flags ("eflags")
165 * The "iret" instruction pops those values off the stack and restores them all
166 * at once. The only problem is that eflags includes the Interrupt Flag which
167 * the Guest can't change: the CPU will simply ignore it when we do an "iret".
168 * So we have to copy eflags from the stack to lguest_data.irq_enabled before
171 * There are two problems with this: firstly, we need to use a register to do
172 * the copy and secondly, the whole thing needs to be atomic. The first
173 * problem is easy to solve: push %eax on the stack so we can use it, and then
174 * restore it at the end just before the real "iret".
176 * The second is harder: copying eflags to lguest_data.irq_enabled will turn
177 * interrupts on before we're finished, so we could be interrupted before we
178 * return to userspace or wherever. Our solution to this is to surround the
179 * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the
180 * Host that it is *never* to interrupt us there, even if interrupts seem to be
188 * Note the %ss: segment prefix here. Normal data accesses use the
189 * "ds" segment, but that will have already been restored for whatever
190 * we're returning to (such as userspace): we can't trust it. The %ss:
191 * prefix makes sure we use the stack segment, which is still valid.
193 movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled