#include #include #include #include #include #include #include /*G:020 * Our story starts with the kernel booting into startup_32 in * arch/x86/kernel/head_32.S. It expects a boot header, which is created by * the bootloader (the Launcher in our case). * * The startup_32 function does very little: it clears the uninitialized global * C variables which we expect to be zero (ie. BSS) and then copies the boot * header and kernel command line somewhere safe. Finally it checks the * 'hardware_subarch' field. This was introduced in 2.6.24 for lguest and Xen: * if it's set to '1' (lguest's assigned number), then it calls us here. * * WARNING: be very careful here! We're running at addresses equal to physical * addesses (around 0), not above PAGE_OFFSET as most code expectes * (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any * data without remembering to subtract __PAGE_OFFSET! * * The .section line puts this code in .init.text so it will be discarded after * boot. */ .section .init.text, "ax", @progbits ENTRY(lguest_entry) /* * We make the "initialization" hypercall now to tell the Host about * us, and also find out where it put our page tables. */ movl $LHCALL_LGUEST_INIT, %eax movl $lguest_data - __PAGE_OFFSET, %ebx int $LGUEST_TRAP_ENTRY /* Set up the initial stack so we can run C code. */ movl $(init_thread_union+THREAD_SIZE),%esp call init_pagetables /* Jumps are relative: we're running __PAGE_OFFSET too low. */ jmp lguest_init+__PAGE_OFFSET /* * Initialize page tables. This creates a PDE and a set of page * tables, which are located immediately beyond __brk_base. The variable * _brk_end is set up to point to the first "safe" location. * Mappings are created both at virtual address 0 (identity mapping) * and PAGE_OFFSET for up to _end. * * FIXME: This code is taken verbatim from arch/x86/kernel/head_32.S: they * don't have a stack at this point, so we can't just use call and ret. */ init_pagetables: #if PTRS_PER_PMD > 1 #define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD) #else #define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD) #endif #define pa(X) ((X) - __PAGE_OFFSET) /* Enough space to fit pagetables for the low memory linear map */ MAPPING_BEYOND_END = \ PAGE_TABLE_SIZE(((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT) << PAGE_SHIFT #ifdef CONFIG_X86_PAE /* * In PAE mode initial_page_table is statically defined to contain * enough entries to cover the VMSPLIT option (that is the top 1, 2 or 3 * entries). The identity mapping is handled by pointing two PGD entries * to the first kernel PMD. * * Note the upper half of each PMD or PTE are always zero at this stage. */ #define KPMDS (((-__PAGE_OFFSET) >> 30) & 3) /* Number of kernel PMDs */ xorl %ebx,%ebx /* %ebx is kept at zero */ movl $pa(__brk_base), %edi movl $pa(initial_pg_pmd), %edx movl $PTE_IDENT_ATTR, %eax 10: leal PDE_IDENT_ATTR(%edi),%ecx /* Create PMD entry */ movl %ecx,(%edx) /* Store PMD entry */ /* Upper half already zero */ addl $8,%edx movl $512,%ecx 11: stosl xchgl %eax,%ebx stosl xchgl %eax,%ebx addl $0x1000,%eax loop 11b /* * End condition: we must map up to the end + MAPPING_BEYOND_END. */ movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp cmpl %ebp,%eax jb 10b 1: addl $__PAGE_OFFSET, %edi movl %edi, pa(_brk_end) shrl $12, %eax movl %eax, pa(max_pfn_mapped) /* Do early initialization of the fixmap area */ movl $pa(initial_pg_fixmap)+PDE_IDENT_ATTR,%eax movl %eax,pa(initial_pg_pmd+0x1000*KPMDS-8) #else /* Not PAE */ page_pde_offset = (__PAGE_OFFSET >> 20); movl $pa(__brk_base), %edi movl $pa(initial_page_table), %edx movl $PTE_IDENT_ATTR, %eax 10: leal PDE_IDENT_ATTR(%edi),%ecx /* Create PDE entry */ movl %ecx,(%edx) /* Store identity PDE entry */ movl %ecx,page_pde_offset(%edx) /* Store kernel PDE entry */ addl $4,%edx movl $1024, %ecx 11: stosl addl $0x1000,%eax loop 11b /* * End condition: we must map up to the end + MAPPING_BEYOND_END. */ movl $pa(_end) + MAPPING_BEYOND_END + PTE_IDENT_ATTR, %ebp cmpl %ebp,%eax jb 10b addl $__PAGE_OFFSET, %edi movl %edi, pa(_brk_end) shrl $12, %eax movl %eax, pa(max_pfn_mapped) /* Do early initialization of the fixmap area */ movl $pa(initial_pg_fixmap)+PDE_IDENT_ATTR,%eax movl %eax,pa(initial_page_table+0xffc) #endif ret /*G:055 * We create a macro which puts the assembler code between lgstart_ and lgend_ * markers. These templates are put in the .text section: they can't be * discarded after boot as we may need to patch modules, too. */ .text #define LGUEST_PATCH(name, insns...) \ lgstart_##name: insns; lgend_##name:; \ .globl lgstart_##name; .globl lgend_##name LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled) LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax) /*G:033 * But using those wrappers is inefficient (we'll see why that doesn't matter * for save_fl and irq_disable later). If we write our routines carefully in * assembler, we can avoid clobbering any registers and avoid jumping through * the wrapper functions. * * I skipped over our first piece of assembler, but this one is worth studying * in a bit more detail so I'll describe in easy stages. First, the routine to * enable interrupts: */ ENTRY(lg_irq_enable) /* * The reverse of irq_disable, this sets lguest_data.irq_enabled to * X86_EFLAGS_IF (ie. "Interrupts enabled"). */ movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled /* * But now we need to check if the Host wants to know: there might have * been interrupts waiting to be delivered, in which case it will have * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we * jump to send_interrupts, otherwise we're done. */ testl $0, lguest_data+LGUEST_DATA_irq_pending jnz send_interrupts /* * One cool thing about x86 is that you can do many things without using * a register. In this case, the normal path hasn't needed to save or * restore any registers at all! */ ret send_interrupts: /* * OK, now we need a register: eax is used for the hypercall number, * which is LHCALL_SEND_INTERRUPTS. * * We used not to bother with this pending detection at all, which was * much simpler. Sooner or later the Host would realize it had to * send us an interrupt. But that turns out to make performance 7 * times worse on a simple tcp benchmark. So now we do this the hard * way. */ pushl %eax movl $LHCALL_SEND_INTERRUPTS, %eax /* * This is a vmcall instruction (same thing that KVM uses). Older * assembler versions might not know the "vmcall" instruction, so we * create one manually here. */ .byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */ /* Put eax back the way we found it. */ popl %eax ret /* * Finally, the "popf" or "restore flags" routine. The %eax register holds the * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're * enabling interrupts again, if it's 0 we're leaving them off. */ ENTRY(lg_restore_fl) /* This is just "lguest_data.irq_enabled = flags;" */ movl %eax, lguest_data+LGUEST_DATA_irq_enabled /* * Now, if the %eax value has enabled interrupts and * lguest_data.irq_pending is set, we want to tell the Host so it can * deliver any outstanding interrupts. Fortunately, both values will * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl" * instruction will AND them together for us. If both are set, we * jump to send_interrupts. */ testl lguest_data+LGUEST_DATA_irq_pending, %eax jnz send_interrupts /* Again, the normal path has used no extra registers. Clever, huh? */ ret /*:*/ /* These demark the EIP range where host should never deliver interrupts. */ .global lguest_noirq_start .global lguest_noirq_end /*M:004 * When the Host reflects a trap or injects an interrupt into the Guest, it * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled, * so the Guest iret logic does the right thing when restoring it. However, * when the Host sets the Guest up for direct traps, such as system calls, the * processor is the one to push eflags onto the stack, and the interrupt bit * will be 1 (in reality, interrupts are always enabled in the Guest). * * This turns out to be harmless: the only trap which should happen under Linux * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc * regions), which has to be reflected through the Host anyway. If another * trap *does* go off when interrupts are disabled, the Guest will panic, and * we'll never get to this iret! :*/ /*G:045 * There is one final paravirt_op that the Guest implements, and glancing at it * you can see why I left it to last. It's *cool*! It's in *assembler*! * * The "iret" instruction is used to return from an interrupt or trap. The * stack looks like this: * old address * old code segment & privilege level * old processor flags ("eflags") * * The "iret" instruction pops those values off the stack and restores them all * at once. The only problem is that eflags includes the Interrupt Flag which * the Guest can't change: the CPU will simply ignore it when we do an "iret". * So we have to copy eflags from the stack to lguest_data.irq_enabled before * we do the "iret". * * There are two problems with this: firstly, we need to use a register to do * the copy and secondly, the whole thing needs to be atomic. The first * problem is easy to solve: push %eax on the stack so we can use it, and then * restore it at the end just before the real "iret". * * The second is harder: copying eflags to lguest_data.irq_enabled will turn * interrupts on before we're finished, so we could be interrupted before we * return to userspace or wherever. Our solution to this is to surround the * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the * Host that it is *never* to interrupt us there, even if interrupts seem to be * enabled. */ ENTRY(lguest_iret) pushl %eax movl 12(%esp), %eax lguest_noirq_start: /* * Note the %ss: segment prefix here. Normal data accesses use the * "ds" segment, but that will have already been restored for whatever * we're returning to (such as userspace): we can't trust it. The %ss: * prefix makes sure we use the stack segment, which is still valid. */ movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled popl %eax iret lguest_noirq_end: