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Date:   Thu, 21 Nov 2019 14:15:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
Message-ID: <20191121131522.GX5671@hirez.programming.kicks-ass.net>
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 <20191121060444.GA55272@gmail.com>
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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 <fenghua.yu@intel.com> 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
that.