[PATCH v5] arm64: mte: allow async MTE to be upgraded to sync on a per-CPU basis

[Will Deacon]

On Fri, Jun 25, 2021 at 03:14:40PM +0100, Szabolcs Nagy wrote:
> The 06/25/2021 13:39, Will Deacon wrote:
> > On Fri, Jun 25, 2021 at 01:01:37PM +0100, Catalin Marinas wrote:
> > > Thanks, that's useful. I guess since the _MTAG_ENABLE tunable is not
> > > ABI, the user app can't rely on what the glibc has configured. Arguably,
> > > since it's driven from outside the application (env), we could say the
> > > same for sysfs, though for the glibc case, the user app is still be able
> > > to override it before the first thread is created (or per-thread). I
> > > assume glibc only issues the prctl() once, not for every new thread.
> 
> note: in the end the tunable is like
> 
> GLIBC_TUNABLES=glibc.mem.tagging=3 ./exe
> 
> not _MTAG_ENABLE.
> 
> and yes the setting comes from outside and glibc
> calls prctl once.
> 
> > > So we can document that the mode requested by the app is an indication,
> > > the system may change it to another value (and back-port documentation
> > > to 5.10). If we get a request from developers to honour a specific mode,
> > > we can add a new PR_MTE_TCF_EXACT bit or something but it's not
> > > essential we do it now.
> > > 
> > > So if we allow the kernel to change the user requested mode (via sysfs),
> > > I think we still have two more issues to clarify:
> > > 
> > > 1. Do we allow only "upgrade" (for some meaning of this) or sysfs can
> > >    downgrade to a less strict mode. I'd go for upgrade here to a
> > >    stricter check as in Peter's patch.
> > > 
> > > 2. Should the sysfs upgrade the PR_MTE_TCF_NONE? _MTAG_ENABLE does that,
> > >    so I'd say yes.
> > > 
> > > Any other thoughts are welcome.
> > 
> > As I mentioned before, I think the sysfs interface should offer:
> > 
> > 	"task"	: Honour whatever the task has asked for (default)
> > 	"async" : Force async on this CPU
> > 	"sync"  : Force sync on this CPU
> > 
> > I don't think we should upgrade PR_MTE_TCF_NONE unless we also have a "none"
> > option in here. I originally suggested that, but in hindsight it feels like
> > a bad idea because a task could SIGILL on migration. So what we're saying is
> > that PR_MTE_TCF_SYNC and PR_MTE_TCF_ASYNC will always enable MTE on success,
> > but the reporting mode is a hint.
> > 
> > I don't think upgrade/downgrade makes a lot of sense given that the sysfs
> > controls can be changed at any point in time. It should just be an override.
> > 
> > This means that we can force async for CPUs where sync mode is horribly
> > slow, whilst honouring the task's request on CPUs which are better
> > implemented.
> 
> i think a user should be able to ask for sync check
> mode for a process and get an error if that's not
> possible.

Hmm. The question then is what do we do if the sysfs override is changed
after the application has started running?

Will