Hi Wei,
On Fri, Jun 22, 2018 at 09:33:04AM +0100, Wei Xu wrote:
> On 2018/6/21 11:54, Will Deacon wrote:
> > On Thu, Jun 21, 2018 at 11:14:28AM +0100, Wei Xu wrote:
> >> On 2018/6/21 10:18, Will Deacon wrote:
> >>> Wei -- does the diff below help at all? Make sure you disable
> >>> CONFIG_KASAN,
> >>> otherwise your kernel will take an age to boot.
> >>
> >> Yes, amazing! This patch resolved the issue.
> >
> > Great...
> >
> >> I have tested 50 times and can not reproduce the issue any more.
> >> Could you please tell more why this patch works?
> >
> > You might need to ask your CPU design team ;)
> >
> > Without this patch, the code in idmap_kpti_install_ng_mappings() sets
> > bit 11 in table descriptors so that we can keep track of which parts of
> > the page table we've visited. With this patch, we don't bother tracking
> > and potentially rewalk parts of the page table (which takes a very long
> > time if KASAN is enabled).
>
> Got it. Thanks!
>
> >
> > The architecture documents I've looked at are clear that bit 11 is IGNORED
> > by the CPU, which:
> >
> > "Indicates that the architecture guarantees that the bit or field is not
> > interpreted or modified by hardware."
> >
> > Please can you double-check that your CPU is indeed ignoring bit 11 in
> > non-leaf (table) descriptors?
>
> Do the non-leaf(table) descriptors mean the table descriptors
> of the section D4.3.1 "VMSAv8-64 translation table level 0, level 1, and
> level 2 descriptor formats"
> in the ARM Architecture Reference Manual ARMv8 for
> ARMv8-A(DDI0487C_a_armv8_arm.pdf)?
>
> If yes, our hardware does ignore it(not interpret or modify).
Ok, thanks for checking.
> Is there any other possible reason cause this?
Perhaps just writing back the table entries is enough to cause the issue,
although I really can't understand why that would be the case. Can you try
the diff below (without my previous change), please?
Will
--->8
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 5f9a73a4452c..e2a8e88f95a0 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -216,7 +216,7 @@ ENDPROC(idmap_cpu_replace_ttbr1)
.endm
.macro __idmap_kpti_put_pgtable_ent_ng, type
- orr \type, \type, #PTE_NG // Same bit for blocks and pages
+ eor \type, \type, #PTE_NG // Same bit for blocks and pages
str \type, [cur_\()\type\()p] // Update the entry and ensure
it
dc civac, cur_\()\type\()p // is visible to all CPUs.
.endm
@@ -298,6 +298,7 @@ skip_pgd:
/* PUD */
walk_puds:
.if CONFIG_PGTABLE_LEVELS > 3
+ eor pgd, pgd, #PTE_NG
pte_to_phys cur_pudp, pgd
add end_pudp, cur_pudp, #(PTRS_PER_PUD * 8)
do_pud: __idmap_kpti_get_pgtable_ent pud
@@ -319,6 +320,7 @@ next_pud:
/* PMD */
walk_pmds:
.if CONFIG_PGTABLE_LEVELS > 2
+ eor pud, pud, #PTE_NG
pte_to_phys cur_pmdp, pud
add end_pmdp, cur_pmdp, #(PTRS_PER_PMD * 8)
do_pmd: __idmap_kpti_get_pgtable_ent pmd
@@ -339,6 +341,7 @@ next_pmd:
/* PTE */
walk_ptes:
+ eor pmd, pmd, #PTE_NG
pte_to_phys cur_ptep, pmd
add end_ptep, cur_ptep, #(PTRS_PER_PTE * 8)
do_pte: __idmap_kpti_get_pgtable_ent pte
