From: Zhang Yanfei <zhangyan...@cn.fujitsu.com>
In head_64.S, a switchover has been used to handle kernel crossing
1G, 512G boundaries.
And commit 8170e6bed465b4b0c7687f93e9948aca4358a33b
x86, 64bit: Use a #PF handler to materialize early mappings on demand
said:
During the switchover in head_64.S, before #PF handler is available,
we use three pages to handle kernel crossing 1G, 512G boundaries with
sharing page by playing games with page aliasing: the same page is
mapped twice in the higher-level tables with appropriate wraparound.
The switchover code is:
104 leaq _text(%rip), %rdi
105 leaq early_level4_pgt(%rip), %rbx
106
107 movq %rdi, %rax
108 shrq $PGDIR_SHIFT, %rax
109
110 leaq (4096 + _KERNPG_TABLE)(%rbx), %rdx
111 movq %rdx, 0(%rbx,%rax,8)
112 movq %rdx, 8(%rbx,%rax,8)
113
114 addq $4096, %rdx
115 movq %rdi, %rax
116 shrq $PUD_SHIFT, %rax
117 andl $(PTRS_PER_PUD-1), %eax
118 movq %rdx, (4096+0)(%rbx,%rax,8)
119 movq %rdx, (4096+8)(%rbx,%rax,8)
120
121 addq $8192, %rbx
122 movq %rdi, %rax
123 shrq $PMD_SHIFT, %rdi
124 addq $(__PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL), %rax
125 leaq (_end - 1)(%rip), %rcx
126 shrq $PMD_SHIFT, %rcx
127 subq %rdi, %rcx
128 incl %ecx
129
130 1:
131 andq $(PTRS_PER_PMD - 1), %rdi
132 movq %rax, (%rbx,%rdi,8)
133 incq %rdi
134 addq $PMD_SIZE, %rax
135 decl %ecx
136 jnz 1b
It seems line 119 has a potential bug there. For example,
the kernel is loaded at physical address 511G+1008M, that is
000000000 111111111 111111000 000000000000000000000
and the kernel _end is 512G+2M, that is
000000001 000000000 000000001 000000000000000000000
So in this example, when using the 2nd page to setup PUD (line 114~119),
rax is 511.
In line 118, we put rdx which is the address of the PMD page (the 3rd page)
into entry 511 of the PUD table. But in line 119, the entry we calculate from
(4096+8)(%rbx,%rax,8) has exceeded the PUD page. IMO, the entry in line
119 should be wraparound into entry 0 of the PUD table.
Sorry for not having a machine with memory exceeding 512GB, so I cannot
test to see if my guess is right or not. Please correct me if I am wrong.
Signed-off-by: Zhang Yanfei <zhangyan...@cn.fujitsu.com>
---
arch/x86/kernel/head_64.S | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 08f7e80..2395d8f 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -116,8 +116,13 @@ startup_64:
shrq $PUD_SHIFT, %rax
andl $(PTRS_PER_PUD-1), %eax
movq %rdx, (4096+0)(%rbx,%rax,8)
+ cmp $511, %rax
+ je 1f
movq %rdx, (4096+8)(%rbx,%rax,8)
-
+ jmp 2f
+1:
+ movq %rdx, (4096)(%rbx)
+2:
addq $8192, %rbx
movq %rdi, %rax
shrq $PMD_SHIFT, %rdi
--
1.7.1
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