Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Fenghua Yu <fenghua.yu@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
	"Luck, Tony" <tony.luck@intel.com>, H Peter Anvin <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	"Li, Xiaoyao" <xiaoyao.li@intel.com>, x86 <x86@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection
Date: Wed, 29 Jul 2020 13:00:33 -0700	[thread overview]
Message-ID: <20200729200033.GJ27751@linux.intel.com> (raw)
In-Reply-To: <20200729194259.GA318576@otcwcpicx6.sc.intel.com>

On Wed, Jul 29, 2020 at 07:42:59PM +0000, Fenghua Yu wrote:
> > Smushing the two into a single option is confusing, e.g. from the table
> > below it's not at all clear what will happen if sld=fatal, both features
> > are supported, and the kernel generates a split lock.
> > 
> > Given that both SLD (per-core, not architectural) and BLD (#DB recursion and
> > inverted DR6 flag) have warts, it would be very nice to enable/disable them
> > independently.  The lock to non-WB behavior for BLD may also be problematic,
> > e.g. maybe it turns out that fixing drivers to avoid locks to non-WB isn't
> > as straightforward as avoiding split locks.
> 
> But the two features are related if both of them are enabled in hardware:
> If a split lock happens, SLD will generate #AC before instruction execution
> and BLD will generate #DB after instruction execution.
> 
> The software needs to make them exclusive. The same kernel option reflects
> the relationship and make them exclusive, e.g. "fatal" enables SLD and
> disables BLD, "warn" does the other way.

Why do they need to be exclusive?  We've already established that BLD catches
things that SLD does not.  What's wrong with running sld=fatal and bld=ratelimit
so that split locks never happen and kill applications, and non-WB locks are
are ratelimited?

Sure, sld==warn with bld!=off is a bit silly, but the kernel can easily handle
that particular case.

> If using two different kernel options, the user needs to give right options
> to make both work, e.g. can the user give this combination
> "split_lock_detect=fatal bus_lock_detect=warn"? What does the combination
> mean?

Split locks are fatal, non-WB locks are logged but not fatal.

> There could be many combinations of the two options, some of them
> are meaningful and some of them aren't. Maintaining the combinations is
> unnecessary complex, right?

Honestly, it seems less complex than deciphering the resulting behavior from
that table.

  sld=off|warn|fatal
  bld=off|warn|ratelimit

As above, sld then could become

  if (sld == warn && bld != off) {
          pr_warn("disabling SLD in favor of BLD\n");
          sld = off;
  }

Everything else should simply work.  The necessary refactoring for SLD should
be minimial as well.