Re: [PATCH v2] riscv: Add tracepoints for SBI calls and returns

[Alexandre Ghiti]

On 13/08/2024 22:26, Conor Dooley wrote:
> On Thu, Mar 21, 2024 at 04:01:25PM -0700, Samuel Holland wrote:
> > These are useful for measuring the latency of SBI calls. The SBI HSM
> > extension is excluded because those functions are called from contexts
> > such as cpuidle where instrumentation is not allowed.
> >
> > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> I seem to have bisected a boot failure on v6.11-rc# to this commit,
> with a crash before the kernel ever gets to output anything - even on
> the bootconsole. For one of these crashes I got the below (which is
> actually U-Boots exception handler), but most of the time - nothing at
> all. I'll try to decodecode that tomorrow and fish out a config, because
> this is either config or toolchain dependant (I saw it in work, with a
> slightly different config etc to what I have here).
>
>
> Starting kernel ...
>   
> Unhandled exception: Load access fault
> EPC: 000000008020e09c RA: 000000008020e510 TVAL: ffffffff81cbf1f0
> EPC: 00000000408a009c RA: 00000000408a0510 reloc adjusted
>   
> Code: 3597 01a4 b583 1f45 5613 0035 9a61 95b2 (618c)
>   
>   
> resetting ...
> reset not supported yet
> ### ERROR ### Please RESET the board ###
>
>
> Cheers,
> Conor.


So I have just hit a similar issue while debugging the syzkaller 
failure. The problem is visible with KASAN, TRACEPOINTS and 
RISCV_ALTERNATIVE_EARLY.

The latter is important since this is where the very early sbi call is 
made in riscv_fill_cpu_mfr_info() 
(https://elixir.bootlin.com/linux/v6.11-rc3/source/arch/riscv/kernel/alternative.c#L32).

And since kernel/sbi.c is compiled with KASAN instrumentation, it fails 
to access the KASAN region as it is not yet set up (the MMU is not up 
yet). So the following fixes the issue:

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 06d407f1b30b..3b926c9444d0 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -31,6 +31,7 @@ endif
 ifdef CONFIG_KASAN
 KASAN_SANITIZE_alternative.o := n
 KASAN_SANITIZE_cpufeature.o := n
+KASAN_SANITIZE_sbi.o := n
 endif
 endif

But I don't like this solution as it removes instrumentation everywhere 
in this file. I'd rather move the __sbi_ecall() into its own file and 
remove the instrumentation here.

@Conor Can you give the diff above a try and see if it fixes your issue?

Thanks,

Alex


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