Re: [EXT] Re: [PATCH v3 03/13] task_isolation: add instruction synchronization memory barrier

[Will Deacon]

On Sun, Apr 19, 2020 at 05:02:01AM +0000, Alex Belits wrote:
> On Wed, 2020-04-15 at 13:44 +0100, Mark Rutland wrote:
> > On Thu, Apr 09, 2020 at 03:17:40PM +0000, Alex Belits wrote:
> > > Some architectures implement memory synchronization instructions
> > > for
> > > instruction cache. Make a separate kind of barrier that calls them.
> > 
> > Modifying the instruction caches requries more than an ISB, and the
> > 'IMB' naming implies you're trying to order against memory accesses,
> > which isn't what ISB (generally) does.
> > 
> > What exactly do you want to use this for?
> 
> I guess, there should be different explanation and naming.
> 
> The intention is to have a separate barrier that causes cache
> synchronization event, for use in architecture-independent code. I am
> not sure, what exactly it should do to be implemented in architecture-
> independent manner, so it probably only makes sense along with a
> regular memory barrier.
> 
> The particular place where I had to use is the code that has to run
> after isolated task returns to the kernel. In the model that I propose
> for task isolation, remote context synchronization is skipped while
> task is in isolated in userspace (it doesn't run kernel, and kernel
> does not modify its userspace code, so it's harmless until entering the
> kernel).

> So it will skip the results of kick_all_cpus_sync() that was
> that was called from flush_icache_range() and other similar places.
> This means that once it's out of userspace, it should only run
> some "safe" kernel entry code, and then synchronize in some manner that
> avoids race conditions with possible IPIs intended for context
> synchronization that may happen at the same time. My next patch in the
> series uses it in that one place.
> 
> Synchronization will have to be implemented without a mandatory
> interrupt because it may be triggered locally, on the same CPU. On ARM,
> ISB is definitely necessary there, however I am not sure, how this
> should look like on x86 and other architectures. On ARM this probably
> still should be combined with a real memory barrier and cache
> synchronization, however I am not entirely sure about details. Would
> it make more sense to run DMB, IC and ISB? 

IIUC, we don't need to do anything on arm64 because taking an exception acts
as a context synchronization event, so I don't think you should try to
expose this as a new barrier macro. Instead, just make it a pre-requisite
that architectures need to ensure this behaviour when entering the kernel
from userspace if they are to select HAVE_ARCH_TASK_ISOLATION.

That way, it's /very/ similar to what we do for MEMBARRIER_SYNC_CORE, the
only real different being that that is concerned with return-to-user rather
than entry-from-user.

See Documentation/features/sched/membarrier-sync-core/arch-support.txt

Will