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authorRusty Russell <rusty@rustcorp.com.au>2007-07-26 10:41:04 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-07-26 11:35:17 -0700
commitf8f0fdcd40449d318f8dc30c1b361b0b7f54134a (patch)
tree09bdfa13377de602bcd0f363a417a93666115c7e /drivers/lguest/core.c
parentbff672e630a015d5b54c8bfb16160b7edc39a57c (diff)
lguest: documentation VI: Switcher
Documentation: The Switcher Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'drivers/lguest/core.c')
-rw-r--r--drivers/lguest/core.c51
1 files changed, 47 insertions, 4 deletions
diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index c0f50b4dd2f1..0a46e8837d9a 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -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