Re: [RFC v2 1/6] x86: introduce kernel restartable sequence

[Nadav Amit]

> On Jan 3, 2019, at 2:48 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> 
>> Okâ Iâll try to think about another solution. Just note that this is just
>> used as a hint to avoid unnecessary lookups. (IOW, nothing will break if the
>> prefix is used.)
> 
> Are you sure actually? 
> 
> The empty prefix could mean 8bit register accesses.
> 
>>> You're doing the equivalent of patching a private system call
>>> into your own kernel without working with upstream, don't do that.
>> 
>> I donât understand this comment though. Can you please explain?
> 
> Instruction encoding = system call ABI
> Upstream = CPU vendors
> 
> Early in Linux's history, naive Linux distribution vendors patched in their own
> private system calls without waiting for upstream to define an ABI, which caused
> endless compatibility problems. These days this is very frowned upon.
> 
>>> Better to find some other solution to do the restart.
>>> How about simply using a per cpu variable? That should be cheaper
>>> anyways.
>> 
>> The problem is that the per-cpu variable needs to be updated after the call
>> is executed, when we are already not in the context of the âinjectedâ code.
>> I can increase it before the call, and decrease it after return - but this
>> can create (in theory) long periods in which the code is âunpatchableâ,
>> increase the code size and slow performance.
>> 
>> Anyhow, Iâll give more thought. Ideas are welcomed.
> 
> Write the address of the instruction into the per cpu variable.

Thanks for the explanations. I donât think it would work (e.g., IRQs). I can
avoid generalizing and just detect the "magic sequenceâ of the code, but let
me give it some more thought.