----- On Nov 10, 2017, at 4:57 PM, Mathieu Desnoyers [email protected] wrote:
> ----- On Nov 10, 2017, at 4:36 PM, Linus Torvalds > [email protected] > wrote: > >> On Fri, Nov 10, 2017 at 1:12 PM, Mathieu Desnoyers >> <[email protected]> wrote: >>> x86 can return to user-space through sysexit and sysretq, which are not >>> core serializing. This breaks expectations from user-space about >>> sequential consistency from a single-threaded self-modifying program >>> point of view in specific migration patterns. >>> >>> Feedback is welcome, >> >> We should check with Intel. I would actually be surprised if the I$ >> can be out of sync with the D$ after a sysretq. It would actually >> break things like "read code from disk" too in theory. > > That core serializing instruction is not that much about I$ vs D$ > consistency, but rather about the processor speculatively executing code > ahead of its retirement point. Ref. Intel Architecture Software Developer's > Manual, Volume 3: System Programming. > > 7.1.3. "Handling Self- and Cross-Modifying Code": > > "The act of a processor writing data into a currently executing code segment > with the intent of > executing that data as code is called self-modifying code. Intel Architecture > processors exhibit > model-specific behavior when executing self-modified code, depending upon how > far ahead of > the current execution pointer the code has been modified. As processor > architectures become > more complex and start to speculatively execute code ahead of the retirement > point (as in the P6 > family processors), the rules regarding which code should execute, pre- or > post-modification, > become blurred. [...]" > > AFAIU, this core serializing instruction seems to be needed for use-cases of > self-modifying code, but not for the initial load of a program from disk, > as the processor has no way to have speculatively executed any of its > instructions. I figured out what you're pointing to: if exec() is executed by a previously running thread, and there is no core serializing instruction between program load and return to user-space, the kernel ends up acting like a JIT, indeed. Therefore, we'd also need to invoke sync_core_before_usermode() after loading the program. Let's wait to hear back from hpa, Thanks, Mathieu > > Hopefully hpa can tell us more about this, > > Thanks, > > Mathieu > > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
