Messages in this thread | | | Date | Wed, 29 Jul 2020 20:35:57 +0000 | From | Fenghua Yu <> | Subject | Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection |
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Hi, Sean,
On Wed, Jul 29, 2020 at 01:00:33PM -0700, Sean Christopherson wrote: > 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?
Sorry if I didn't explain bus lock and split lock detections clearly before.
There are two causes of bus locks: 1. a locked access across cache line boundary: this is split lock. 2. a locked access to non-WB memory.
BLD detects both causes and SLD only detects the first one, i.e. BLD can detect both split lock AND lock to non-WB memory.
If sld=fatal and bld=ratelimit (both sld and bld are enabled in hw), a split lock always generates #AC and kills the app and bld will never have a chance to trigger #DB for split lock. So effectively the combination makes the kernel to take two different actions after detecting a bus lock: if the bus lock comes from a split lock, fatal (sld); if the bus lock comes from lock to non-WB memory, ratelimit (bld). Seems this is not a useful combination and is not what the user really wants to do because the user wants ratelimit for BLD, right?
> > 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.
Similar here: bus lock from a split lock is fatal (sld triggers #AC) and bus lock from lock to non-WB mem is warn (bld triggers #DB). Seems not what the user really wants, right?
Thanks.
-Fenghua
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