Messages in this thread | | | Date | Wed, 29 Jul 2020 13:00:33 -0700 | From | Sean Christopherson <> | Subject | Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection |
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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.
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