[Xen-devel] [PATCH 2/3] x86/cpa: Use pte_attrs instead of pte_flags on CPA/set_p.._wb/wc operations.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

When using the paravirt interface, most of the page operations are wrapped
in the pvops interface. The one that is not is the pte_flags. The reason
being that for most cases, the "raw" PTE flag values for baremetal and whatever
pvops platform is running (in this case) - share the same bit meaning.

Except for PAT. Under Linux, the PAT MSR is written to be:

          PAT4                 PAT0
+---+----+----+----+-----+----+----+
 WC | WC | WB | UC | UC- | WC | WB |  <= Linux
+---+----+----+----+-----+----+----+
 WC | WT | WB | UC | UC- | WT | WB |  <= BIOS
+---+----+----+----+-----+----+----+
 WC | WP | WC | UC | UC- | WT | WB |  <= Xen
+---+----+----+----+-----+----+----+

The lookup of this index table translates to looking up
Bit 7, Bit 4, and Bit 3 of PTE:

 PAT/PSE (bit 7) ... PCD (bit 4) .. PWT (bit 3).

If all bits are off, then we are using PAT0. If bit 3 turned on,
then we are using PAT1, if bit 3 and bit 4, then PAT2..

Back to the PAT MSR table:

As you can see, the PAT1 translates to PAT4 under Xen. Under Linux
we only use PAT0, PAT1, and PAT2 for the caching as:

 WB = none (so PAT0)
 WC = PWT (bit 3 on)
 UC = PWT | PCD (bit 3 and 4 are on).

But to make it work with Xen, we end up doing for WC a translation:

 PWT (so bit 3 on) --> PAT (so bit 7 is on) and clear bit 3

And to translate back (when the paravirt pte_val is used) we would:

 PAT (bit 7 on) --> PWT (bit 3 on) and clear bit 7.

This works quite well, except if code uses the pte_flags, as pte_flags
reads the raw value and does not go through the paravirt. Which means
that if (when running under Xen):

 1) we allocate some pages.
 2) call set_pages_array_wc, which ends up calling:
     __page_change_att_set_clr(.., __pgprot(__PAGE_WC),  /* set */
                                 , __pgprot(__PAGE_MASK), /* clear */
    which ends up reading the _raw_ PTE flags and _only_ look at the
    _PTE_FLAG_MASK contents with __PAGE_MASK cleared (0x18) and
    __PAGE_WC (0x8) set.

     read raw *pte -> 0x67
     *pte = 0x67 & ^0x18 | 0x8
     *pte = 0x67 & 0xfffffe7 | 0x8
     *pte = 0x6f

   [now set_pte_atomic is called, and 0x6f is written in, but under
    xen_make_pte, the bit 3 is translated to bit 7, so it ends up
    writting 0xa7, which is correct]

 3) do something to them.
 4) call set_pages_array_wb
     __page_change_att_set_clr(.., __pgprot(__PAGE_WB),  /* set */
                                 , __pgprot(__PAGE_MASK), /* clear */
    which ends up reading the _raw_ PTE and _only_ look at the
    _PTE_FLAG_MASK contents with _PAGE_MASK cleared (0x18) and
    __PAGE_WB (0x0) set:

     read raw *pte -> 0xa7
     *pte = 0xa7 & &0x18 | 0
     *pte = 0xa7 & 0xfffffe7 | 0
     *pte = 0xa7

   [we check whether the old PTE is different from the new one

    if (pte_val(old_pte) != pte_val(new_pte)) {
        set_pte_atomic(kpte, new_pte);
        ...

   and find out that 0xA7 == 0xA7 so we do not write the new PTE value in]

   End result is that we failed at removing the WC caching bit!

 5) free them.
   [and have pages with PAT4 (bit 7) set, so other subsystems end up using
    the pages that have the write combined bit set resulting in crashes. 
Yikes!].

The fix, which this patch proposes, is to wrap the pte_pgprot in the CPA
code with newly introduced pte_attrs which can go through the pvops interface
to get the "emulated" value instead of the raw. Naturally if CONFIG_PARAVIRT is
not set, it would end calling native_pte_val.

The other way to fix this is by wrapping pte_flags and go through the pvops
interface and it really is the Right Thing to do.  The problem is, that past
experience with mprotect stuff demonstrates that it be really expensive in inner
loops, and pte_flags() is used in some very perf-critical areas.

Example code to run this and see the various mysterious subsystems/applications
crashing

MODULE_AUTHOR("Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>");
MODULE_DESCRIPTION("wb_to_wc_and_back");
MODULE_LICENSE("GPL");
MODULE_VERSION(WB_TO_WC);

static int thread(void *arg)
{
        struct page *a[MAX_PAGES];
        unsigned int i, j;
        do {
                for (j = 0, i = 0;i < MAX_PAGES; i++, j++) {
                        a[i] = alloc_page(GFP_KERNEL);
                        if (!a[i])
                                break;
                }
                set_pages_array_wc(a, j);
                set_current_state(TASK_INTERRUPTIBLE);
                schedule_timeout_interruptible(HZ);
                for (i = 0; i < j; i++) {
                        unsigned long *addr = page_address(a[i]);
                        if (addr) {
                                memset(addr, 0xc2, PAGE_SIZE);
                        }
                }
                set_pages_array_wb(a, j);
                for (i = 0; i< MAX_PAGES; i++) {
                        if (a[i])
                                __free_page(a[i]);
                        a[i] = NULL;
                }
        } while (!kthread_should_stop());
        return 0;
}
static struct task_struct *t;
static int __init wb_to_wc_init(void)
{
        t = kthread_run(thread, NULL, "wb_to_wc_and_back");
        return 0;
}
static void __exit wb_to_wc_exit(void)
{
        if (t)
                kthread_stop(t);
}
module_init(wb_to_wc_init);
module_exit(wb_to_wc_exit);

This fixes RH BZ #742032
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: stable@xxxxxxxxxx
---
 arch/x86/include/asm/pgtable.h |    5 +++++
 arch/x86/mm/pageattr.c         |    2 +-
 2 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 18601c8..34027b0 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -349,6 +349,11 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, 
pgprot_t newprot)
        return __pgprot(preservebits | addbits);
 }
 
+static inline pgprot_t pte_attrs(pte_t pte)
+{
+       return __pgprot(pte_val(pte) & PTE_FLAGS_MASK);
+}
+
 #define pte_pgprot(x) __pgprot(pte_flags(x) & PTE_FLAGS_MASK)
 
 #define canon_pgprot(p) __pgprot(massage_pgprot(p))
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index f9e5267..efa8040 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -651,7 +651,7 @@ repeat:
 
        if (level == PG_LEVEL_4K) {
                pte_t new_pte;
-               pgprot_t new_prot = pte_pgprot(old_pte);
+               pgprot_t new_prot = pte_attrs(old_pte);
                unsigned long pfn = pte_pfn(old_pte);
 
                pgprot_val(new_prot) &= ~pgprot_val(cpa->mask_clr);
-- 
1.7.7.1


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