Commit f8f0fdcd authored by Rusty Russell's avatar Rusty Russell Committed by Linus Torvalds

lguest: documentation VI: Switcher

Documentation: The Switcher
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bff672e6
......@@ -393,46 +393,89 @@ static void set_ts(void)
write_cr0(cr0|8);
}
/*S:010
* We are getting close to the Switcher.
*
* Remember that each CPU has two pages which are visible to the Guest when it
* runs on that CPU. This has to contain the state for that Guest: we copy the
* state in just before we run the Guest.
*
* Each Guest has "changed" flags which indicate what has changed in the Guest
* since it last ran. We saw this set in interrupts_and_traps.c and
* segments.c.
*/
static void copy_in_guest_info(struct lguest *lg, struct lguest_pages *pages)
{
/* Copying all this data can be quite expensive. We usually run the
* same Guest we ran last time (and that Guest hasn't run anywhere else
* meanwhile). If that's not the case, we pretend everything in the
* Guest has changed. */
if (__get_cpu_var(last_guest) != lg || lg->last_pages != pages) {
__get_cpu_var(last_guest) = lg;
lg->last_pages = pages;
lg->changed = CHANGED_ALL;
}
/* These are pretty cheap, so we do them unconditionally. */
/* These copies are pretty cheap, so we do them unconditionally: */
/* Save the current Host top-level page directory. */
pages->state.host_cr3 = __pa(current->mm->pgd);
/* Set up the Guest's page tables to see this CPU's pages (and no
* other CPU's pages). */
map_switcher_in_guest(lg, pages);
/* Set up the two "TSS" members which tell the CPU what stack to use
* for traps which do directly into the Guest (ie. traps at privilege
* level 1). */
pages->state.guest_tss.esp1 = lg->esp1;
pages->state.guest_tss.ss1 = lg->ss1;
/* Copy direct trap entries. */
/* Copy direct-to-Guest trap entries. */
if (lg->changed & CHANGED_IDT)
copy_traps(lg, pages->state.guest_idt, default_idt_entries);
/* Copy all GDT entries but the TSS. */
/* Copy all GDT entries which the Guest can change. */
if (lg->changed & CHANGED_GDT)
copy_gdt(lg, pages->state.guest_gdt);
/* If only the TLS entries have changed, copy them. */
else if (lg->changed & CHANGED_GDT_TLS)
copy_gdt_tls(lg, pages->state.guest_gdt);
/* Mark the Guest as unchanged for next time. */
lg->changed = 0;
}
/* Finally: the code to actually call into the Switcher to run the Guest. */
static void run_guest_once(struct lguest *lg, struct lguest_pages *pages)
{
/* This is a dummy value we need for GCC's sake. */
unsigned int clobber;
/* Copy the guest-specific information into this CPU's "struct
* lguest_pages". */
copy_in_guest_info(lg, pages);
/* Put eflags on stack, lcall does rest: suitable for iret return. */
/* Now: we push the "eflags" register on the stack, then do an "lcall".
* This is how we change from using the kernel code segment to using
* the dedicated lguest code segment, as well as jumping into the
* Switcher.
*
* The lcall also pushes the old code segment (KERNEL_CS) onto the
* stack, then the address of this call. This stack layout happens to
* exactly match the stack of an interrupt... */
asm volatile("pushf; lcall *lguest_entry"
/* This is how we tell GCC that %eax ("a") and %ebx ("b")
* are changed by this routine. The "=" means output. */
: "=a"(clobber), "=b"(clobber)
/* %eax contains the pages pointer. ("0" refers to the
* 0-th argument above, ie "a"). %ebx contains the
* physical address of the Guest's top-level page
* directory. */
: "0"(pages), "1"(__pa(lg->pgdirs[lg->pgdidx].pgdir))
/* We tell gcc that all these registers could change,
* which means we don't have to save and restore them in
* the Switcher. */
: "memory", "%edx", "%ecx", "%edi", "%esi");
}
/*:*/
/*H:030 Let's jump straight to the the main loop which runs the Guest.
* Remember, this is called by the Launcher reading /dev/lguest, and we keep
......
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