Re: [PATCH v5 00/13] riscv: improve boot time isa extensions handling

[Guenter Roeck]

Hi,

On Sun, Jan 29, 2023 at 01:28:43AM +0800, Jisheng Zhang wrote:
> Generally, riscv ISA extensions are fixed for any specific hardware
> platform, so a hart's features won't change after booting, this
> chacteristic makes it straightforward to use a static branch to check
> a specific ISA extension is supported or not to optimize performance.
> 
> However, some ISA extensions such as SVPBMT and ZICBOM are handled
> via. the alternative sequences.
> 
> Basically, for ease of maintenance, we prefer to use static branches
> in C code, but recently, Samuel found that the static branch usage in
> cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As
> Samuel pointed out, "Having a static branch in cpu_relax() is
> problematic because that function is widely inlined, including in some
> quite complex functions like in the VDSO. A quick measurement shows
> this static branch is responsible by itself for around 40% of the jump
> table."
> 
> Samuel's findings pointed out one of a few downsides of static branches
> usage in C code to handle ISA extensions detected at boot time:
> static branch's metadata in the __jump_table section, which is not
> discarded after ISA extensions are finalized, wastes some space.
> 
> I want to try to solve the issue for all possible dynamic handling of
> ISA extensions at boot time. Inspired by Mark[2], this patch introduces
> riscv_has_extension_*() helpers, which work like static branches but
> are patched using alternatives, thus the metadata can be freed after
> patching.
> 

This patch series results in boot failures when trying to boot the
qemu sifive_u emulation. There are many log messages along the line of

[    0.000000] WARNING: CPU: 0 PID: 0 at arch/riscv/kernel/patch.c:63 patch_insn_write+0x222/0x2f6
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Tainted: G        W          6.2.0-rc7-next-20230210 #1
[    0.000000] Hardware name: SiFive HiFive Unleashed A00 (DT)
[    0.000000] epc : patch_insn_write+0x222/0x2f6
[    0.000000]  ra : patch_insn_write+0x21e/0x2f6
[    0.000000] epc : ffffffff800068c6 ra : ffffffff800068c2 sp : ffffffff81803df0
[    0.000000]  gp : ffffffff81a1ab78 tp : ffffffff81814f80 t0 : ffffffffffffe000
[    0.000000]  t1 : 0000000000000001 t2 : 4c45203a76637369 s0 : ffffffff81803e40
[    0.000000]  s1 : 0000000000000004 a0 : 0000000000000000 a1 : ffffffffffffffff
[    0.000000]  a2 : 0000000000000004 a3 : 0000000000000000 a4 : 0000000000000001
[    0.000000]  a5 : 0000000000000000 a6 : 0000000000000000 a7 : 0000000052464e43
[    0.000000]  s2 : ffffffff80b4889c s3 : 000000000000082c s4 : ffffffff80b48828
[    0.000000]  s5 : 0000000000000828 s6 : ffffffff8131a0a0 s7 : 0000000000000fff
[    0.000000]  s8 : 0000000008000200 s9 : ffffffff8131a520 s10: 0000000000000018
[    0.000000]  s11: 000000000000000b t3 : 0000000000000001 t4 : 000000000000000d
[    0.000000]  t5 : ffffffffd8180000 t6 : ffffffff81803bc8
[    0.000000] status: 0000000200000100 badaddr: 0000000000000000 cause: 0000000000000003
[    0.000000] [<ffffffff800068c6>] patch_insn_write+0x222/0x2f6
[    0.000000] [<ffffffff80006a36>] patch_text_nosync+0xc/0x2a
[    0.000000] [<ffffffff80003b86>] riscv_cpufeature_patch_func+0x52/0x98
[    0.000000] [<ffffffff80003348>] _apply_alternatives+0x46/0x86
[    0.000000] [<ffffffff80c02d36>] apply_boot_alternatives+0x3c/0xfa
[    0.000000] [<ffffffff80c03ad8>] setup_arch+0x584/0x5b8
[    0.000000] [<ffffffff80c0075a>] start_kernel+0xa2/0x8f8
[    0.000000] irq event stamp: 0
[    0.000000] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
[    0.000000] hardirqs last disabled at (0): [<0000000000000000>] 0x0
[    0.000000] softirqs last  enabled at (0): [<0000000000000000>] 0x0
[    0.000000] softirqs last disabled at (0): [<0000000000000000>] 0x0
[    0.000000] ---[ end trace 0000000000000000 ]---
[    0.000000] ------------[ cut here ]------------

then qemu hangs until the session is aborted.

Similar messages are also seen with the "virt" emulation, but there the boot
does not hang but fails to find a root device.

Guenter



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# first bad commit: [9daca9a5b9ac35361ce2d8d5ec10b19b7048d6cd] Merge patch series "riscv: improve boot time isa extensions handling"