Re: [Xen-devel] [PATCHv2] x86/xen: avoid m2p lookup when setting early page table entries

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 22/06/16 11:54, David Vrabel wrote:
> On 21/06/16 20:31, Boris Ostrovsky wrote:
>> On 06/21/2016 12:09 PM, David Vrabel wrote:
>>> When page tables entries are set using xen_set_pte_init() during early
>>> boot there is no page fault handler that could handle a fault when
>>> performing an M2P lookup.
>>>
>>> In 64 guest (usually dom0) early_ioremap() would fault in
>>> xen_set_pte_init() because an M2P lookup faults because the MFN is in
>>> MMIO space and not mapped in the M2P.  This lookup is done to see if
>>> the PFN in in the range used for the initial page table pages, so that
>>> the PTE may be set as read-only.
>>>
>>> The M2P lookup can be avoided by moving the check (and clear of RW)
>>> earlier when the PFN is still available.
> [...]
>>> --- a/arch/x86/xen/mmu.c
>>> +++ b/arch/x86/xen/mmu.c
>>> @@ -1562,7 +1562,7 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t 
>>> pte)
>>>     return pte;
>>>  }
>>>  #else /* CONFIG_X86_64 */
>>> -static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
>>> +static pteval_t __init mask_rw_pte(pteval_t pte)
>>>  {
>>>     unsigned long pfn;
>>>  
>>> @@ -1577,10 +1577,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t 
>>> pte)
>>>      * page tables for mapping the p2m list, too, and page tables MUST be
>>>      * mapped read-only.
>>>      */
>>> -   pfn = pte_pfn(pte);
>>> +   pfn = (pte & PTE_PFN_MASK) >> PAGE_SHIFT;
>>
>> Is it obvious that we are holding valid PFN at this point? It wasn't
>> immediately obvious to me so I wonder whether a comment stating this
>> would be useful here (yes, you mention it in the commit messages).
> 
> I don't understand what you mean by a "valid PFN"?
> 
> This is only called from xen_make_pte_init() which is for converting
> ptevals containing PFNs to MFNs.  Did that answer your question?
> 
> Would it be clearer if I just inlined the two functions like so:

It would help if I included the right diff.

8<---------------------
x86/xen: avoid m2p lookup when setting early page table entries

When page tables entries are set using xen_set_pte_init() during early
boot there is no page fault handler that could handle a fault when
performing an M2P lookup.

In 64 guest (usually dom0) early_ioremap() would fault in
xen_set_pte_init() because an M2P lookup faults because the MFN is in
MMIO space and not mapped in the M2P.  This lookup is done to see if
the PFN in in the range used for the initial page table pages, so that
the PTE may be set as read-only.

The M2P lookup can be avoided by moving the check (and clear of RW)
earlier when the PFN is still available.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Tested-by: Keven Moraga <kmoragas@xxxxxxxxxx>
---
v3:
- fold mask_rw_pte()/mask_rw_pteval() into their callers.

v2:
- Remove __init annotation from xen_make_pte_init() since
  PV_CALLEE_SAVE_REGS_THUNK always puts the thunk in .text.

- mask_rw_pte() -> mask_rw_pteval() for x86-64.
---
 arch/x86/xen/mmu.c | 76
+++++++++++++++++++++++++-----------------------------
 1 file changed, 35 insertions(+), 41 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 478a2de..64d8f0b 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1551,41 +1551,6 @@ static void xen_pgd_free(struct mm_struct *mm,
pgd_t *pgd)
 #endif
 }

-#ifdef CONFIG_X86_32
-static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
-{
-       /* If there's an existing pte, then don't allow _PAGE_RW to be set */
-       if (pte_val_ma(*ptep) & _PAGE_PRESENT)
-               pte = __pte_ma(((pte_val_ma(*ptep) & _PAGE_RW) | ~_PAGE_RW) &
-                              pte_val_ma(pte));
-
-       return pte;
-}
-#else /* CONFIG_X86_64 */
-static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
-{
-       unsigned long pfn;
-
-       if (xen_feature(XENFEAT_writable_page_tables) ||
-           xen_feature(XENFEAT_auto_translated_physmap) ||
-           xen_start_info->mfn_list >= __START_KERNEL_map)
-               return pte;
-
-       /*
-        * Pages belonging to the initial p2m list mapped outside the default
-        * address range must be mapped read-only. This region contains the
-        * page tables for mapping the p2m list, too, and page tables MUST be
-        * mapped read-only.
-        */
-       pfn = pte_pfn(pte);
-       if (pfn >= xen_start_info->first_p2m_pfn &&
-           pfn < xen_start_info->first_p2m_pfn + xen_start_info->nr_p2m_frames)
-               pte = __pte_ma(pte_val_ma(pte) & ~_PAGE_RW);
-
-       return pte;
-}
-#endif /* CONFIG_X86_64 */
-
 /*
  * Init-time set_pte while constructing initial pagetables, which
  * doesn't allow RO page table pages to be remapped RW.
@@ -1600,13 +1565,41 @@ static pte_t __init mask_rw_pte(pte_t *ptep,
pte_t pte)
  * so always write the PTE directly and rely on Xen trapping and
  * emulating any updates as necessary.
  */
-static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
+__visible pte_t xen_make_pte_init(pteval_t pte)
 {
-       if (pte_mfn(pte) != INVALID_P2M_ENTRY)
-               pte = mask_rw_pte(ptep, pte);
-       else
-               pte = __pte_ma(0);
+#ifdef CONFIG_X86_64
+       unsigned long pfn;
+
+       /*
+        * Pages belonging to the initial p2m list mapped outside the default
+        * address range must be mapped read-only. This region contains the
+        * page tables for mapping the p2m list, too, and page tables MUST be
+        * mapped read-only.
+        */
+       pfn = (pte & PTE_PFN_MASK) >> PAGE_SHIFT;
+       if (xen_start_info->mfn_list < __START_KERNEL_map &&
+           pfn >= xen_start_info->first_p2m_pfn &&
+           pfn < xen_start_info->first_p2m_pfn + xen_start_info->nr_p2m_frames)
+               pte &= ~_PAGE_RW;
+#endif
+       pte = pte_pfn_to_mfn(pte);

+       if ((pte & PTE_PFN_MASK) >> PAGE_SHIFT == INVALID_P2M_ENTRY)
+               pte = 0;
+
+       return native_make_pte(pte);
+}
+PV_CALLEE_SAVE_REGS_THUNK(xen_make_pte_init);
+
+static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
+{
+#ifdef CONFIG_X86_32
+       /* If there's an existing pte, then don't allow _PAGE_RW to be set */
+       if (pte_mfn(pte) != INVALID_P2M_ENTRY
+           && pte_val_ma(*ptep) & _PAGE_PRESENT)
+               pte = __pte_ma(((pte_val_ma(*ptep) & _PAGE_RW) | ~_PAGE_RW) &
+                              pte_val_ma(pte));
+#endif
        native_set_pte(ptep, pte);
 }

@@ -2407,6 +2400,7 @@ static void __init xen_post_allocator_init(void)
        pv_mmu_ops.alloc_pud = xen_alloc_pud;
        pv_mmu_ops.release_pud = xen_release_pud;
 #endif
+       pv_mmu_ops.make_pte = PV_CALLEE_SAVE(xen_make_pte);

 #ifdef CONFIG_X86_64
        pv_mmu_ops.write_cr3 = &xen_write_cr3;
@@ -2455,7 +2449,7 @@ static const struct pv_mmu_ops xen_mmu_ops
__initconst = {
        .pte_val = PV_CALLEE_SAVE(xen_pte_val),
        .pgd_val = PV_CALLEE_SAVE(xen_pgd_val),

-       .make_pte = PV_CALLEE_SAVE(xen_make_pte),
+       .make_pte = PV_CALLEE_SAVE(xen_make_pte_init),
        .make_pgd = PV_CALLEE_SAVE(xen_make_pgd),

 #ifdef CONFIG_X86_PAE
-- 
2.1.4


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