Re: [PATCH -next v4 3/7] arm64: add support for machine check error safe

[Mark Rutland <mark.rutland@arm.com>]

On Thu, May 26, 2022 at 11:36:41AM +0800, Tong Tiangen wrote:
> 
> 
> 在 2022/5/25 16:30, Mark Rutland 写道:
> > On Thu, May 19, 2022 at 02:29:54PM +0800, Tong Tiangen wrote:
> > > 
> > > 
> > > 在 2022/5/13 23:26, Mark Rutland 写道:
> > > > On Wed, Apr 20, 2022 at 03:04:14AM +0000, Tong Tiangen wrote:
> > > > > During the processing of arm64 kernel hardware memory errors(do_sea()), if
> > > > > the errors is consumed in the kernel, the current processing is panic.
> > > > > However, it is not optimal.
> > > > > 
> > > > > Take uaccess for example, if the uaccess operation fails due to memory
> > > > > error, only the user process will be affected, kill the user process
> > > > > and isolate the user page with hardware memory errors is a better choice.
> > > > 
> > > > Conceptually, I'm fine with the idea of constraining what we do for a
> > > > true uaccess, but I don't like the implementation of this at all, and I
> > > > think we first need to clean up the arm64 extable usage to clearly
> > > > distinguish a uaccess from another access.
> > > 
> > > OK,using EX_TYPE_UACCESS and this extable type could be recover, this is
> > > more reasonable.
> > 
> > Great.
> > 
> > > For EX_TYPE_UACCESS_ERR_ZERO, today we use it for kernel accesses in a
> > > couple of cases, such as
> > > get_user/futex/__user_cache_maint()/__user_swpX_asm(),
> > 
> > Those are all user accesses.
> > 
> > However, __get_kernel_nofault() and __put_kernel_nofault() use
> > EX_TYPE_UACCESS_ERR_ZERO by way of __{get,put}_mem_asm(), so we'd need to
> > refactor that code to split the user/kernel cases higher up the callchain.
> > 
> > > your suggestion is:
> > > get_user continues to use EX_TYPE_UACCESS_ERR_ZERO and the other cases use
> > > new type EX_TYPE_FIXUP_ERR_ZERO?
> > 
> > Yes, that's the rough shape. We could make the latter EX_TYPE_KACCESS_ERR_ZERO
> > to be clearly analogous to EX_TYPE_UACCESS_ERR_ZERO, and with that I susepct we
> > could remove EX_TYPE_FIXUP.
> > 
> > Thanks,
> > Mark.
> According to your suggestion, i think the definition is like this:
> 
> #define EX_TYPE_NONE                    0
> #define EX_TYPE_FIXUP                   1    --> delete
> #define EX_TYPE_BPF                     2
> #define EX_TYPE_UACCESS_ERR_ZERO        3
> #define EX_TYPE_LOAD_UNALIGNED_ZEROPAD  4
> #define EX_TYPE_UACCESS		        xx   --> add
> #define EX_TYPE_KACCESS_ERR_ZERO        xx   --> add
> [The value defined by the macro here is temporary]

Almost; you don't need to add EX_TYPE_UACCESS here, as you can use
EX_TYPE_UACCESS_ERR_ZERO for that.

We already have:

| #define _ASM_EXTABLE_UACCESS_ERR(insn, fixup, err)		\
|         _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, wzr)

... and we can add:

| #define _ASM_EXTABLE_UACCESS(insn, fixup)			\
|         _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, wzr, wzr)


... and maybe we should use 'xzr' rather than 'wzr' for clarity.

> There are two points to modify:
> 
> 1、_get_kernel_nofault() and __put_kernel_nofault()  using
> EX_TYPE_KACCESS_ERR_ZERO, Other positions using EX_TYPE_UACCESS_ERR_ZERO
> keep unchanged.

That sounds right to me. This will require refactoring __raw_{get,put}_mem()
and __{get,put}_mem_asm().

> 2、delete EX_TYPE_FIXUP.
> 
> There is no doubt about others. As for EX_TYPE_FIXUP, I think it needs to be
> retained, _cond_extable(EX_TYPE_FIXUP) is still in use in assembler.h.

We use _cond_extable for cache maintenance uaccesses, so those should be moved
over to to EX_TYPE_UACCESS_ERR_ZERO. We can rename _cond_extable to
_cond_uaccess_extable for clarity.

That will require restructuring asm-extable.h a bit. If that turns out to be
painful I'm happy to take a look.

Thanks,
Mark.