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> 7 8/*G:020 9 10 * Our story starts with the bzImage: booting starts at startup_32 in 11 * arch/x86/boot/compressed/head_32.S. This merely uncompresses the real 12 * kernel in place and then jumps into it: startup_32 in 13 * arch/x86/kernel/head_32.S. Both routines expects a boot header in the %esi 14 * register, which is created by the bootloader (the Launcher in our case). 15 * 16 * The startup_32 function does very little: it clears the uninitialized global 17 * C variables which we expect to be zero (ie. BSS) and then copies the boot 18 * header and kernel command line somewhere safe, and populates some initial 19 * page tables. Finally it checks the 'hardware_subarch' field. This was 20 * introduced in 2.6.24 for lguest and Xen: if it's set to '1' (lguest's 21 * assigned number), then it calls us here. 22 * 23 * WARNING: be very careful here! We're running at addresses equal to physical 24 * addresses (around 0), not above PAGE_OFFSET as most code expects 25 * (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any 26 * data without remembering to subtract __PAGE_OFFSET! 27 * 28 * The .section line puts this code in .init.text so it will be discarded after 29 * boot. 30 */ 31.section .init.text, "ax", @progbits 32ENTRY(lguest_entry) 33 /* 34 * We make the "initialization" hypercall now to tell the Host where 35 * our lguest_data struct is. 36 */ 37 movl $LHCALL_LGUEST_INIT, %eax 38 movl $lguest_data - __PAGE_OFFSET, %ebx 39 int $LGUEST_TRAP_ENTRY 40 41 /* Now turn our pagetables on; setup by arch/x86/kernel/head_32.S. */ 42 movl $LHCALL_NEW_PGTABLE, %eax 43 movl $(initial_page_table - __PAGE_OFFSET), %ebx 44 int $LGUEST_TRAP_ENTRY 45 46 /* Set up the initial stack so we can run C code. */ 47 movl $(init_thread_union+THREAD_SIZE),%esp 48 49 /* Jumps are relative: we're running __PAGE_OFFSET too low. */ 50 jmp lguest_init+__PAGE_OFFSET 51 52/*G:055 53 * We create a macro which puts the assembler code between lgstart_ and lgend_ 54 * markers. These templates are put in the .text section: they can't be 55 * discarded after boot as we may need to patch modules, too. 56 */ 57.text 58#define LGUEST_PATCH(name, insns...) \ 59 lgstart_##name: insns; lgend_##name:; \ 60 .globl lgstart_##name; .globl lgend_##name 61 62LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled) 63LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax) 64 65/*G:033 66 * But using those wrappers is inefficient (we'll see why that doesn't matter 67 * for save_fl and irq_disable later). If we write our routines carefully in 68 * assembler, we can avoid clobbering any registers and avoid jumping through 69 * the wrapper functions. 70 * 71 * I skipped over our first piece of assembler, but this one is worth studying 72 * in a bit more detail so I'll describe in easy stages. First, the routine to 73 * enable interrupts: 74 */ 75ENTRY(lg_irq_enable) 76 /* 77 * The reverse of irq_disable, this sets lguest_data.irq_enabled to 78 * X86_EFLAGS_IF (ie. "Interrupts enabled"). 79 */ 80 movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled 81 /* 82 * But now we need to check if the Host wants to know: there might have 83 * been interrupts waiting to be delivered, in which case it will have 84 * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we 85 * jump to send_interrupts, otherwise we're done. 86 */ 87 cmpl $0, lguest_data+LGUEST_DATA_irq_pending 88 jnz send_interrupts 89 /* 90 * One cool thing about x86 is that you can do many things without using 91 * a register. In this case, the normal path hasn't needed to save or 92 * restore any registers at all! 93 */ 94 ret 95send_interrupts: 96 /* 97 * OK, now we need a register: eax is used for the hypercall number, 98 * which is LHCALL_SEND_INTERRUPTS. 99 * 100 * We used not to bother with this pending detection at all, which was 101 * much simpler. Sooner or later the Host would realize it had to 102 * send us an interrupt. But that turns out to make performance 7 103 * times worse on a simple tcp benchmark. So now we do this the hard 104 * way. 105 */ 106 pushl %eax 107 movl $LHCALL_SEND_INTERRUPTS, %eax 108 /* This is the actual hypercall trap. */ 109 int $LGUEST_TRAP_ENTRY 110 /* Put eax back the way we found it. */ 111 popl %eax 112 ret 113 114/* 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. 118 */ 119ENTRY(lg_restore_fl) 120 /* This is just "lguest_data.irq_enabled = flags;" */ 121 movl %eax, lguest_data+LGUEST_DATA_irq_enabled 122 /* 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. 129 */ 130 testl lguest_data+LGUEST_DATA_irq_pending, %eax 131 jnz send_interrupts 132 /* Again, the normal path has used no extra registers. Clever, huh? */ 133 ret 134/*:*/ 135 136/* These demark the EIP where host should never deliver interrupts. */ 137.global lguest_noirq_iret 138 139/*M:004 140 * When the Host reflects a trap or injects an interrupt into the Guest, it 141 * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled, 142 * so the Guest iret logic does the right thing when restoring it. However, 143 * when the Host sets the Guest up for direct traps, such as system calls, the 144 * processor is the one to push eflags onto the stack, and the interrupt bit 145 * will be 1 (in reality, interrupts are always enabled in the Guest). 146 * 147 * This turns out to be harmless: the only trap which should happen under Linux 148 * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc 149 * regions), which has to be reflected through the Host anyway. If another 150 * trap *does* go off when interrupts are disabled, the Guest will panic, and 151 * we'll never get to this iret! 152:*/ 153 154/*G:045 155 * There is one final paravirt_op that the Guest implements, and glancing at it 156 * you can see why I left it to last. It's *cool*! It's in *assembler*! 157 * 158 * The "iret" instruction is used to return from an interrupt or trap. The 159 * stack looks like this: 160 * old address 161 * old code segment & privilege level 162 * old processor flags ("eflags") 163 * 164 * The "iret" instruction pops those values off the stack and restores them all 165 * at once. The only problem is that eflags includes the Interrupt Flag which 166 * the Guest can't change: the CPU will simply ignore it when we do an "iret". 167 * So we have to copy eflags from the stack to lguest_data.irq_enabled before 168 * we do the "iret". 169 * 170 * There are two problems with this: firstly, we can't clobber any registers 171 * and secondly, the whole thing needs to be atomic. The first problem 172 * is solved by using "push memory"/"pop memory" instruction pair for copying. 173 * 174 * The second is harder: copying eflags to lguest_data.irq_enabled will turn 175 * interrupts on before we're finished, so we could be interrupted before we 176 * return to userspace or wherever. Our solution to this is to tell the 177 * Host that it is *never* to interrupt us there, even if interrupts seem to be 178 * enabled. (It's not necessary to protect pop instruction, since 179 * data gets updated only after it completes, so we only need to protect 180 * one instruction, iret). 181 */ 182ENTRY(lguest_iret) 183 pushl 2*4(%esp) 184 /* 185 * Note the %ss: segment prefix here. Normal data accesses use the 186 * "ds" segment, but that will have already been restored for whatever 187 * we're returning to (such as userspace): we can't trust it. The %ss: 188 * prefix makes sure we use the stack segment, which is still valid. 189 */ 190 popl %ss:lguest_data+LGUEST_DATA_irq_enabled 191lguest_noirq_iret: 192 iret 193