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lguest: update commentry

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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>
This commit is contained in:
Rusty Russell
2009-07-30 16:03:45 -06:00
parent 2e04ef7691
commit a91d74a3c4
11 changed files with 399 additions and 112 deletions
Download Patch File Download Diff File Expand all files Collapse all files

86
drivers/lguest/page_tables.c
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@@ -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 @@ static DEFINE_PER_CPU(pte_t *, switcher_pte_pages);
/*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 @@ static pte_t *spte_addr(struct lg_cpu *cpu, pgd_t spgd, unsigned long vaddr)
}
/*
* 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 @@ static unsigned long gpgd_addr(struct lg_cpu *cpu, unsigned long vaddr)
}
#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 @@ static unsigned long gpmd_addr(pgd_t gpgd, unsigned long vaddr)
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 @@ static unsigned long gpte_addr(struct lg_cpu *cpu,
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 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
pte_t gpte;
pte_t *spte;
/* Mid level for PAE. */
#ifdef CONFIG_X86_PAE
pmd_t *spmd;
pmd_t gpmd;
@@ -391,6 +395,8 @@ bool demand_page(struct lg_cpu *cpu, unsigned long vaddr, int errcode)
*/
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 @@ void pin_page(struct lg_cpu *cpu, unsigned long vaddr)
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 @@ static void release_pgd(pgd_t *spgd)
}
#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 @@ void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
/* ... 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 @@ static unsigned long setup_pagetables(struct lguest *lg,
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 @@ static unsigned long setup_pagetables(struct lguest *lg,
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 @@ static unsigned long setup_pagetables(struct lguest *lg,
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 @@ static unsigned long setup_pagetables(struct lguest *lg,
#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 @@ int init_guest_pagetable(struct lguest *lg)
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 @@ int init_guest_pagetable(struct lguest *lg)
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 @@ void map_switcher_in_guest(struct lg_cpu *cpu, struct lguest_pages *pages)
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;