Re: [PATCH v2 -next] riscv: mm: remove redundant trampoline PGD for 64bit
From: Palmer Dabbelt
Date: Fri Aug 13 2021 - 18:08:31 EST
On Mon, 02 Aug 2021 05:43:02 PDT (-0700), alex@xxxxxxxx wrote:
Hi Nanyong,
Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
Remove redundant trampoline PGD for 64bit and add more comment
for why 32bit systems need trampoline PGD.
There was a patch and discussion similar to this,refer to
the link [1][2].
The trampoline PGD is redundant for 64bit systems because:
1. The early PGD covers the entire kernel mapping. Directly
loading early PGD can achieve the result in boot stage.
A more trampoline PGD makes code hard to understand.
2. Directly loading early PGD is safe in 64bit systems since
the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
which has a very big gap with RAM address.It won't fall into
the corner case that 32bit system worrys.
3. Remove redundant trampoline PGD can benefit to code maintaince,
because 64bit systems have more page table levels.For example:
If we want to support SV48 which has 4 page table levels, we have
to add a trampoline_pud and insert it before trampoline_pmd.
Reference link:
[1]https://lore.kernel.org/linux-riscv/20190325092234.5451-4-anup.patel@xxxxxxx/
[2]https://lkml.org/lkml/2019/3/28/147
Signed-off-by: Nanyong Sun <sunnanyong@xxxxxxxxxx>
Reviewed-by: Alexandre Ghiti <alex@xxxxxxxx>
I have not reviewed this patch yet! :)
Reviewed-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Reviewed-by: weiyongjun <weiyongjun1@xxxxxxxxxx>
---
v2 changes:
Adjust codes based on the review suggestions from Alex Ghiti,
make codes more readable.
---
arch/riscv/kernel/head.S | 46 ++++++++++++++++++++++++++--------------
arch/riscv/mm/init.c | 16 ++++----------
2 files changed, 34 insertions(+), 28 deletions(-)
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index fce5184b22c3..3816aa5edc69 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -89,29 +89,52 @@ relocate:
add ra, ra, a1
/* Point stvec to virtual address of intruction after satp write */
- la a2, 1f
+#ifdef CONFIG_64BIT
+ la a2, load_done
+#else
+ la a2, load_kernel_pgd
+#endif
add a2, a2, a1
csrw CSR_TVEC, a2
- /* Compute satp for kernel page tables, but don't load it yet */
+ /* Compute satp for kernel page tables.
+ * For 64bit systems, load it and trap to stvec.
+ * For 32bit systems, don't load it yet.
+ */
srl a2, a0, PAGE_SHIFT
li a1, SATP_MODE
or a2, a2, a1
/*
+ * Before writing satp, we need a full fence here because
setup_vm() just
+ * wrote these PTEs and we need to ensure the new translations
are in use.
+ */
+ sfence.vma
+#ifndef CONFIG_64BIT
+ /*
+ * 32bit systems need firstly loading a trampoline to handle a
corner
+ * case where load address range overlaps kernel virtual address
range.
* Load trampoline page directory, which will cause us to trap to
- * stvec if VA != PA, or simply fall through if VA == PA. We need a
- * full fence here because setup_vm() just wrote these PTEs and
we need
- * to ensure the new translations are in use.
+ * stvec if VA != PA, or simply fall through if VA == PA.
*/
la a0, trampoline_pg_dir
XIP_FIXUP_OFFSET a0
srl a0, a0, PAGE_SHIFT
or a0, a0, a1
- sfence.vma
csrw CSR_SATP, a0
+#endif
.align 2
-1:
+load_kernel_pgd:
+ /*
+ * Switch to kernel page tables. A full fence is necessary
in order to
+ * avoid using the trampoline translations, which are only
correct for
+ * the first superpage. Fetching the fence is guarnteed to work
+ * because that first superpage is translated the same way.
+ */
+ csrw CSR_SATP, a2
+ sfence.vma
+
+load_done:
/* Set trap vector to spin forever to help debug */
la a0, .Lsecondary_park
csrw CSR_TVEC, a0
I suppose stvec was set this way to catch any problem with early_pg_dir,
you moved that and then this defeats this original purpose.
Essentially.
The specific issue is that the JTAG debug spec is defined (or at least
was when I was using it, it's been years since I've needed to do that)
in terms of committed instructions. Thus if you end up in a position
where the processer is unable to commit an instruction you also lose the
ability to do anything meaningful with the debugger, thus essentially
locking up the system.
The most common way to end up in a situation where the processor is
unable to commit an instruction is to have a fault with an invalid trap
vector: maybe dangling from M-mode, the last boot, reset, whatever.
Then as soon as you take a trap the system locks up. Any trap before we
have a working trap handler is a bug, but it's way harder to debug
things when the debugger doesn't function.
There is of course no way to fundamentally prevent these sort of
no-commitable-instruction situations, but I got into the habbit of just
setting up a trivial trap entry point ASAP -- it probably took a dozen
rounds of trying to debug the debugger only to realize it was per spec
to hang, but that idiom eventually crept into pretty much everything.
Not sure if the debug spec is still written this way (or if debuggers
respect it), as I haven't had to use one in a while.
@@ -122,15 +145,6 @@ relocate:
la gp, __global_pointer$
.option pop
- /*
- * Switch to kernel page tables. A full fence is necessary in
order to
- * avoid using the trampoline translations, which are only
correct for
- * the first superpage. Fetching the fence is guarnteed to work
- * because that first superpage is translated the same way.
- */
- csrw CSR_SATP, a2
- sfence.vma
-
ret
#endif /* CONFIG_MMU */
#ifdef CONFIG_SMP
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index ac48742fa6fc..306fcb2334fa 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
EXPORT_SYMBOL(pfn_base);
pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
+#ifndef CONFIG_64BIT
pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
+#endif /* CONFIG_64BIT */
As stated in Documentation/process/coding-style.rst, it is better to use
__maybe_unused rather than #ifdefs.
static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
#ifdef CONFIG_XIP_KERNEL
+#ifndef CONFIG_64BIT
#define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
+#endif /* CONFIG_64BIT */
Using __maybe_unused allows to remove those too.
#define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
#define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
#endif /* CONFIG_XIP_KERNEL */
@@ -300,13 +304,11 @@ static void __init create_pte_mapping(pte_t *ptep,
#ifndef __PAGETABLE_PMD_FOLDED
-static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata
__aligned(PAGE_SIZE);
#ifdef CONFIG_XIP_KERNEL
-#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
#define fixmap_pmd ((pmd_t *)XIP_FIXUP(fixmap_pmd))
#define early_pmd ((pmd_t *)XIP_FIXUP(early_pmd))
#endif /* CONFIG_XIP_KERNEL */
@@ -585,16 +587,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
/* Setup fixmap PMD */
create_pmd_mapping(fixmap_pmd, FIXADDR_START,
(uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
- /* Setup trampoline PGD and PMD */
- create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
- (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
-#ifdef CONFIG_XIP_KERNEL
- create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
- kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
-#else
- create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
- kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
-#endif
#else
/* Setup trampoline PGD */
create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
Overall this version adds more complexity to assembly code than I
thought, but I don't see any way to improve that (which does not mean
there isn't!).
Thanks,
Alex
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