* Our story starts with the bzImage: booting starts at startup_32 in
* arch/x86/boot/compressed/head_32.S. This merely uncompresses the real
* kernel in place and then jumps into it: startup_32 in
* arch/x86/kernel/head_32.S. Both routines expects a boot header in the %esi
* register, 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, and populates some initial
* page tables. 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
* addresses (around 0), not above PAGE_OFFSET as most code expects
* (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
.section .init.text, "ax", @progbits
* We make the "initialization" hypercall now to tell the Host where
* our lguest_data struct is.
movl $LHCALL_LGUEST_INIT, %eax
movl $lguest_data - __PAGE_OFFSET, %ebx
/* Now turn our pagetables on; setup by arch/x86/kernel/head_32.S. */
movl $LHCALL_NEW_PGTABLE, %eax
movl $(initial_page_table - __PAGE_OFFSET), %ebx
/* Set up the initial stack so we can run C code. */
/* Jumps are relative: we're running __PAGE_OFFSET too low. */
* 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.
#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)
* 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:
* 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.
cmpl $0, lguest_data+LGUEST_DATA_irq_pending
* 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!
* 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
movl $LHCALL_SEND_INTERRUPTS, %eax
/* This is the actual hypercall trap. */
/* Put eax back the way we found it. */
* 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.
/* 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
/* Again, the normal path has used no extra registers. Clever, huh? */
/* These demark the EIP where host should never deliver interrupts. */
* 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!
* 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 can't clobber any registers
* and secondly, the whole thing needs to be atomic. The first problem
* is solved by using "push memory"/"pop memory" instruction pair for copying.
* 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 tell the
* Host that it is *never* to interrupt us there, even if interrupts seem to be
* enabled. (It's not necessary to protect pop instruction, since
* data gets updated only after it completes, so we only need to protect
* one instruction, iret).
* 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.