Re: [PATCH -next v4 4/7] arm64: add copy_{to, from}_user to machine check safe

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Apr 20, 2022 at 03:04:15AM +0000, Tong Tiangen wrote:
> Add copy_{to, from}_user() to machine check safe.
> 
> If copy fail due to hardware memory error, only the relevant processes are
> affected, so killing the user process and isolate the user page with
> hardware memory errors is a more reasonable choice than kernel panic.

Just to make sure I understand - we can only recover if the fault is in
a user page. That is, for a copy_from_user(), we can only handle the
faults in the source address, not the destination.

> diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> index 34e317907524..480cc5ac0a8d 100644
> --- a/arch/arm64/lib/copy_from_user.S
> +++ b/arch/arm64/lib/copy_from_user.S
> @@ -25,7 +25,7 @@
>  	.endm
>  
>  	.macro strb1 reg, ptr, val
> -	strb \reg, [\ptr], \val
> +	USER_MC(9998f, strb \reg, [\ptr], \val)
>  	.endm

So if I got the above correctly, why do we need an exception table entry
for the store to the kernel address?

-- 
Catalin