Re: 32bit kernel is broken for Linux-5.1-rc1 due to GCC cmodel=medlow

[Mike Rapoport <rppt@linux.ibm.com>]

Hi,

On Fri, Mar 22, 2019 at 11:46:24AM +0530, Anup Patel wrote:
> Hi Palmer,
> 
> The 32bit kernel booting is broken for Linux-5.1-rc1 due to GCC cmodel=medlow
> affecting setup_vm() movement from kernel/setup.c to mm/init.c.
> 
> There is no issue with 64bit kernel booting.
> 
> The "[PATCH v2 2/5] RISC-V: Make setup_vm() independent of GCC code model"
> fixes this issue.
> 
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1959102.html
> 
> If possible please include above patch as Linux-5.1-rc1 fix.

I'm not convinced that it's the best solution. Multiple __load_pa() and
__load_va() conversions make code unreadable.
Is there any reason swapper_pg_dir cannot be setup after 'relocate'? It'll
save a lot of churn for the current fix and for the addition of 4K mappings

I've drafted a patch that separates trampoline_pd_dir and swapper_pg_dir
setup, if it works, the __load_pa() and __load_va() conversions can be
applied only to the trampoline initialization. I don't have riscv hardware,
so it's compile tested only.

From d1f6f68012c84e188711954f58d63c0a3ba005c5 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.ibm.com>
Date: Fri, 22 Mar 2019 12:34:31 +0200
Subject: [PATCH] riscv: split trampoline_pg_dir and swapper_pg_dir
 initialization

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
 arch/riscv/kernel/head.S |  7 ++-----
 arch/riscv/mm/init.c     | 33 +++++++++++++++++++++++++--------
 2 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index fe884cd..fe0fc70 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -62,8 +62,9 @@ clear_bss_done:
 
 	/* Initialize page tables and relocate to virtual addresses */
 	la sp, init_thread_union + THREAD_SIZE
-	call setup_vm
+	call setup_trampoline
 	call relocate
+	call setup_vm
 
 	/* Restore C environment */
 	la tp, init_task
@@ -117,10 +118,6 @@ relocate:
 .option norelax
 	la gp, __global_pointer$
 .option pop
-
-	/* Switch to kernel page tables */
-	csrw sptbr, a2
-
 	ret
 
 .Lsecondary_start:
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index b379a75..1742763 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -187,11 +187,6 @@ asmlinkage void __init setup_vm(void)
 	BUG_ON((pa % (PAGE_SIZE * PTRS_PER_PTE)) != 0);
 
 #ifndef __PAGETABLE_PMD_FOLDED
-	trampoline_pg_dir[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN((uintptr_t)trampoline_pmd),
-			__pgprot(_PAGE_TABLE));
-	trampoline_pmd[0] = pfn_pmd(PFN_DOWN(pa), prot);
-
 	for (i = 0; i < (-PAGE_OFFSET)/PGDIR_SIZE; ++i) {
 		size_t o = (PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD + i;
 
@@ -209,9 +204,6 @@ asmlinkage void __init setup_vm(void)
 		pfn_pmd(PFN_DOWN((uintptr_t)fixmap_pte),
 				__pgprot(_PAGE_TABLE));
 #else
-	trampoline_pg_dir[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
-		pfn_pgd(PFN_DOWN(pa), prot);
-
 	for (i = 0; i < (-PAGE_OFFSET)/PGDIR_SIZE; ++i) {
 		size_t o = (PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD + i;
 
@@ -223,4 +215,29 @@ asmlinkage void __init setup_vm(void)
 		pfn_pgd(PFN_DOWN((uintptr_t)fixmap_pte),
 				__pgprot(_PAGE_TABLE));
 #endif
+	csr_write(sptbr, virt_to_pfn(swapper_pg_dir) | SATP_MODE);
+}
+
+asmlinkage void __init setup_trampoline(void)
+{
+	extern char _start;
+	uintptr_t pa = (uintptr_t) &_start;
+	pgprot_t prot = __pgprot(pgprot_val(PAGE_KERNEL) | _PAGE_EXEC);
+
+	va_pa_offset = PAGE_OFFSET - pa;
+	pfn_base = PFN_DOWN(pa);
+
+	/* Sanity check alignment and size */
+	BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
+	BUG_ON((pa % (PAGE_SIZE * PTRS_PER_PTE)) != 0);
+
+#ifndef __PAGETABLE_PMD_FOLDED
+	trampoline_pg_dir[pgd_index(PAGE_OFFSET)] =
+		pfn_pgd(PFN_DOWN((uintptr_t)trampoline_pmd),
+			__pgprot(_PAGE_TABLE));
+	trampoline_pmd[0] = pfn_pmd(PFN_DOWN(pa), prot);
+
+#else
+	trampoline_pg_dir[pgd_index(PAGE_OFFSET)] = pfn_pgd(PFN_DOWN(pa), prot);
+#endif
 }
-- 
2.7.4

 
> Regards,
> Anup
> 

-- 
Sincerely yours,
Mike.


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