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riscv: add nommu support

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The kernel runs in M-mode without using page tables, and thus can't run
bare metal without help from additional firmware.

Most of the patch is just stubbing out code not needed without page
tables, but there is an interesting detail in the signals implementation:

 - The normal RISC-V syscall ABI only implements rt_sigreturn as VDSO
   entry point, but the ELF VDSO is not supported for nommu Linux.
   We instead copy the code to call the syscall onto the stack.

In addition to enabling the nommu code a new defconfig for a small
kernel image that can run in nommu mode on qemu is also provided, to run
a kernel in qemu you can use the following command line:

qemu-system-riscv64 -smp 2 -m 64 -machine virt -nographic \
	-kernel arch/riscv/boot/loader \
	-drive file=rootfs.ext2,format=raw,id=hd0 \
	-device virtio-blk-device,drive=hd0

Contains contributions from Damien Le Moal <Damien.LeMoal@wdc.com>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
[paul.walmsley@sifive.com: updated to apply; add CONFIG_MMU guards
 around PCI_IOBASE definition to fix build issues; fixed checkpatch
 issues; move the PCI_IO_* and VMEMMAP address space macros along
 with the others; resolve sparse warning]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
This commit is contained in:
Christoph Hellwig
2019-10-28 13:10:41 +01:00
committed by Paul Walmsley
parent 9e80635619
commit 6bd33e1ece
23 changed files with 254 additions and 71 deletions
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6
arch/riscv/kernel/head.S
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@@ -109,8 +109,10 @@ clear_bss_done:
la sp, init_thread_union + THREAD_SIZE
mv a0, s1
call setup_vm
#ifdef CONFIG_MMU
la a0, early_pg_dir
call relocate
#endif /* CONFIG_MMU */
/* Restore C environment */
la tp, init_task
@@ -121,6 +123,7 @@ clear_bss_done:
call parse_dtb
tail start_kernel
#ifdef CONFIG_MMU
relocate:
/* Relocate return address */
li a1, PAGE_OFFSET
@@ -171,6 +174,7 @@ relocate:
sfence.vma
ret
#endif /* CONFIG_MMU */
.Lsecondary_start:
#ifdef CONFIG_SMP
@@ -196,9 +200,11 @@ relocate:
beqz tp, .Lwait_for_cpu_up
fence
#ifdef CONFIG_MMU
/* Enable virtual memory and relocate to virtual address */
la a0, swapper_pg_dir
call relocate
#endif
tail smp_callin
#endif