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SubjectRe: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter
On Thu, Nov 21, 2019 at 02:01:53PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 21, 2019 at 07:04:44AM +0100, Ingo Molnar wrote:
> > * Fenghua Yu <> wrote:
> > > + split_lock_detect
> > > + [X86] Enable split lock detection
> > > + This is a real time or debugging feature. When enabled
> > > + (and if hardware support is present), atomic
> > > + instructions that access data across cache line
> > > + boundaries will result in an alignment check exception.
> > > + When triggered in applications the kernel will send
> > > + SIGBUS. The kernel will panic for a split lock in
> > > + OS code.
> >
> > It would be really nice to be able to enable/disable this runtime as
> > well, has this been raised before, and what was the conclusion?
> It has, previous versions had that. Somehow a lot of things went missing
> and we're back to a broken neutered useless mess.
> The problem appears to be that due to hardware design the feature cannot
> be virtualized, and instead of then disabling it when a VM runs/exists
> they just threw in the towel and went back to useless mode.. :-(
> This feature MUST be default enabled, otherwise everything will
> be/remain broken and we'll end up in the situation where you can't use
> it even if you wanted to.
> Imagine the BIOS/EFI/firmware containing an #AC exception. At that point
> the feature becomes useless, because you cannot enable it without your
> machine dying.
> Now, from long and painful experience we all know that if a BIOS can be
> wrong, it will be. Therefore this feature will be/is useless as
> presented.
> And I can't be arsed to look it up, but we've been making this very same
> argument since very early (possible the very first) version.
> So this version goes straight into the bit bucket. Please try again.

Also, just to remind everyone why we really want this. Split lock is a
potent, unprivileged, DoS vector.

It works nicely across guests and everything. Furthermore no sane
software should have #AC, because RISC machines have been throwing
alignment checks on stupid crap like that forever.

And even on x86, where it 'works' it has been a performance nightmare
for pretty much ever since we lost the Front Side Bus or something like

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