lguest: update commentry
Every so often, after code shuffles, I need to go through and unbitrot
the Lguest Journey (see drivers/lguest/README). Since we now use RCU in
a simple form in one place I took the opportunity to expand that explanation.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index 3da902e..a8d0aee 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -29,10 +29,10 @@
/*H:300
* The Page Table Code
*
- * We use two-level page tables for the Guest. If you're not entirely
- * comfortable with virtual addresses, physical addresses and page tables then
- * I recommend you review arch/x86/lguest/boot.c's "Page Table Handling" (with
- * diagrams!).
+ * We use two-level page tables for the Guest, or three-level with PAE. If
+ * you're not entirely comfortable with virtual addresses, physical addresses
+ * and page tables then I recommend you review arch/x86/lguest/boot.c's "Page
+ * Table Handling" (with diagrams!).
*
* The Guest keeps page tables, but we maintain the actual ones here: these are
* called "shadow" page tables. Which is a very Guest-centric name: these are
@@ -52,9 +52,8 @@
:*/
/*
- * 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is
- * conveniently placed at the top 4MB, so it uses a separate, complete PTE
- * page.
+ * The Switcher uses the complete top PTE page. That's 1024 PTE entries (4MB)
+ * or 512 PTE entries with PAE (2MB).
*/
#define SWITCHER_PGD_INDEX (PTRS_PER_PGD - 1)
@@ -81,7 +80,8 @@
/*H:320
* The page table code is curly enough to need helper functions to keep it
- * clear and clean.
+ * clear and clean. The kernel itself provides many of them; one advantage
+ * of insisting that the Guest and Host use the same CONFIG_PAE setting.
*
* There are two functions which return pointers to the shadow (aka "real")
* page tables.
@@ -155,7 +155,7 @@
}
/*
- * These two functions just like the above two, except they access the Guest
+ * These functions are just like the above two, except they access the Guest
* page tables. Hence they return a Guest address.
*/
static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
@@ -165,6 +165,7 @@
}
#ifdef CONFIG_X86_PAE
+/* Follow the PGD to the PMD. */
static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
{
unsigned long gpage = pgd_pfn(gpgd) << PAGE_SHIFT;
@@ -172,6 +173,7 @@
return gpage + pmd_index(vaddr) * sizeof(pmd_t);
}
+/* Follow the PMD to the PTE. */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pmd_t gpmd, unsigned long vaddr)
{
@@ -181,6 +183,7 @@
return gpage + pte_index(vaddr) * sizeof(pte_t);
}
#else
+/* Follow the PGD to the PTE (no mid-level for !PAE). */
static unsigned long gpte_addr(struct lg_cpu *cpu,
pgd_t gpgd, unsigned long vaddr)
{
@@ -314,6 +317,7 @@
pte_t gpte;
pte_t *spte;
+ /* Mid level for PAE. */
#ifdef CONFIG_X86_PAE
pmd_t *spmd;
pmd_t gpmd;
@@ -391,6 +395,8 @@
*/
gpte_ptr = gpte_addr(cpu, gpgd, vaddr);
#endif
+
+ /* Read the actual PTE value. */
gpte = lgread(cpu, gpte_ptr, pte_t);
/* If this page isn't in the Guest page tables, we can't page it in. */
@@ -507,6 +513,7 @@
if (!page_writable(cpu, vaddr) && !demand_page(cpu, vaddr, 2))
kill_guest(cpu, "bad stack page %#lx", vaddr);
}
+/*:*/
#ifdef CONFIG_X86_PAE
static void release_pmd(pmd_t *spmd)
@@ -543,7 +550,11 @@
}
#else /* !CONFIG_X86_PAE */
-/*H:450 If we chase down the release_pgd() code, it looks like this: */
+/*H:450
+ * If we chase down the release_pgd() code, the non-PAE version looks like
+ * this. The PAE version is almost identical, but instead of calling
+ * release_pte it calls release_pmd(), which looks much like this.
+ */
static void release_pgd(pgd_t *spgd)
{
/* If the entry's not present, there's nothing to release. */
@@ -898,17 +909,21 @@
/* ... throw it away. */
release_pgd(lg->pgdirs[pgdir].pgdir + idx);
}
+
#ifdef CONFIG_X86_PAE
+/* For setting a mid-level, we just throw everything away. It's easy. */
void guest_set_pmd(struct lguest *lg, unsigned long pmdp, u32 idx)
{
guest_pagetable_clear_all(&lg->cpus[0]);
}
#endif
-/*
- * Once we know how much memory we have we can construct simple identity (which
+/*H:505
+ * To get through boot, we construct simple identity page mappings (which
* set virtual == physical) and linear mappings which will get the Guest far
- * enough into the boot to create its own.
+ * enough into the boot to create its own. The linear mapping means we
+ * simplify the Guest boot, but it makes assumptions about their PAGE_OFFSET,
+ * as you'll see.
*
* We lay them out of the way, just below the initrd (which is why we need to
* know its size here).
@@ -944,6 +959,10 @@
linear = (void *)pgdir - linear_pages * PAGE_SIZE;
#ifdef CONFIG_X86_PAE
+ /*
+ * And the single mid page goes below that. We only use one, but
+ * that's enough to map 1G, which definitely gets us through boot.
+ */
pmds = (void *)linear - PAGE_SIZE;
#endif
/*
@@ -957,13 +976,14 @@
return -EFAULT;
}
- /*
- * The top level points to the linear page table pages above.
- * We setup the identity and linear mappings here.
- */
#ifdef CONFIG_X86_PAE
+ /*
+ * Make the Guest PMD entries point to the corresponding place in the
+ * linear mapping (up to one page worth of PMD).
+ */
for (i = j = 0; i < mapped_pages && j < PTRS_PER_PMD;
i += PTRS_PER_PTE, j++) {
+ /* FIXME: native_set_pmd is overkill here. */
native_set_pmd(&pmd, __pmd(((unsigned long)(linear + i)
- mem_base) | _PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
@@ -971,18 +991,36 @@
return -EFAULT;
}
+ /* One PGD entry, pointing to that PMD page. */
set_pgd(&pgd, __pgd(((u32)pmds - mem_base) | _PAGE_PRESENT));
+ /* Copy it in as the first PGD entry (ie. addresses 0-1G). */
if (copy_to_user(&pgdir[0], &pgd, sizeof(pgd)) != 0)
return -EFAULT;
+ /*
+ * And the third PGD entry (ie. addresses 3G-4G).
+ *
+ * FIXME: This assumes that PAGE_OFFSET for the Guest is 0xC0000000.
+ */
if (copy_to_user(&pgdir[3], &pgd, sizeof(pgd)) != 0)
return -EFAULT;
#else
+ /*
+ * The top level points to the linear page table pages above.
+ * We setup the identity and linear mappings here.
+ */
phys_linear = (unsigned long)linear - mem_base;
for (i = 0; i < mapped_pages; i += PTRS_PER_PTE) {
pgd_t pgd;
+ /*
+ * Create a PGD entry which points to the right part of the
+ * linear PTE pages.
+ */
pgd = __pgd((phys_linear + i * sizeof(pte_t)) |
(_PAGE_PRESENT | _PAGE_RW | _PAGE_USER));
+ /*
+ * Copy it into the PGD page at 0 and PAGE_OFFSET.
+ */
if (copy_to_user(&pgdir[i / PTRS_PER_PTE], &pgd, sizeof(pgd))
|| copy_to_user(&pgdir[pgd_index(PAGE_OFFSET)
+ i / PTRS_PER_PTE],
@@ -992,8 +1030,8 @@
#endif
/*
- * We return the top level (guest-physical) address: remember where
- * this is.
+ * We return the top level (guest-physical) address: we remember where
+ * this is to write it into lguest_data when the Guest initializes.
*/
return (unsigned long)pgdir - mem_base;
}
@@ -1031,7 +1069,9 @@
lg->pgdirs[0].pgdir = (pgd_t *)get_zeroed_page(GFP_KERNEL);
if (!lg->pgdirs[0].pgdir)
return -ENOMEM;
+
#ifdef CONFIG_X86_PAE
+ /* For PAE, we also create the initial mid-level. */
pgd = lg->pgdirs[0].pgdir;
pmd_table = (pmd_t *) get_zeroed_page(GFP_KERNEL);
if (!pmd_table)
@@ -1040,11 +1080,13 @@
set_pgd(pgd + SWITCHER_PGD_INDEX,
__pgd(__pa(pmd_table) | _PAGE_PRESENT));
#endif
+
+ /* This is the current page table. */
lg->cpus[0].cpu_pgd = 0;
return 0;
}
-/* When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
+/*H:508 When the Guest calls LHCALL_LGUEST_INIT we do more setup. */
void page_table_guest_data_init(struct lg_cpu *cpu)
{
/* We get the kernel address: above this is all kernel memory. */
@@ -1105,12 +1147,16 @@
pmd_t switcher_pmd;
pmd_t *pmd_table;
+ /* FIXME: native_set_pmd is overkill here. */
native_set_pmd(&switcher_pmd, pfn_pmd(__pa(switcher_pte_page) >>
PAGE_SHIFT, PAGE_KERNEL_EXEC));
+ /* Figure out where the pmd page is, by reading the PGD, and converting
+ * it to a virtual address. */
pmd_table = __va(pgd_pfn(cpu->lg->
pgdirs[cpu->cpu_pgd].pgdir[SWITCHER_PGD_INDEX])
<< PAGE_SHIFT);
+ /* Now write it into the shadow page table. */
native_set_pmd(&pmd_table[SWITCHER_PMD_INDEX], switcher_pmd);
#else
pgd_t switcher_pgd;