Re: [RFC PATCH 0/2] x86: Fix missing core serialization on migration

[Mathieu Desnoyers]

----- On Nov 14, 2017, at 12:03 PM, Avi Kivity avi@xxxxxxxxxxxx wrote:

> On 11/14/2017 06:49 PM, Mathieu Desnoyers wrote:
>> ----- On Nov 14, 2017, at 11:08 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
>>
>>> On Tue, Nov 14, 2017 at 05:05:41PM +0100, Peter Zijlstra wrote:
>>>> On Tue, Nov 14, 2017 at 03:17:12PM +0000, Mathieu Desnoyers wrote:
>>>>> I've tried to create a small single-threaded self-modifying loop in
>>>>> user-space to trigger a trace cache or speculative execution quirk,
>>>>> but I have not succeeded yet. I suspect that I would need to know
>>>>> more about the internals of the processor architecture to create the
>>>>> right stalls that would allow speculative execution to move further
>>>>> ahead, and trigger an incoherent execution flow. Ideas on how to
>>>>> trigger this would be welcome.
>>>> I thought the whole problem was per definition multi-threaded.
>>>>
>>>> Single-threaded stuff can't get out of sync with itself; you'll always
>>>> observe your own stores.
>>> And even if you could, you can always execute a local serializing
>>> instruction like CPUID to force things.
>> What I'm trying to reproduce is something that breaks in single-threaded
>> case if I explicitly leave out the CPUID core serializing instruction
>> when doing code modification on upcoming code, in a loop.
>>
>> AFAIU, Intel requires a core serializing instruction to be issued even
>> in single-threaded scenarios between code update and execution, to ensure
>> that speculative execution does not observe incoherent code. Now the
>> question we all have for Intel is: is this requirement too strong, or
>> required by reality ?
>>
> 
> In single-threaded execution, a jump is enough.
> 
> "As processor microarchitectures become more complex and start to
> speculatively execute code ahead of the retire-
> ment point (as in P6 and more recent processor families), the rules
> regarding which code should execute, pre- or
> post-modification, become blurred. To write self-modifying code and
> ensure that it is compliant with current and
> future versions of the IA-32 architectures, use one of the following
> coding options:
> 
> (* OPTION 1 *)
> Store modified code (as data) into code segment;
> Jump to new code or an intermediate location;
> Execute new code;"

Good point, so this is likely why I was having trouble reproducing the
single-threaded self-modifying code incoherent case. I did have a branch
in there.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com