Messages in this thread | | | Date | Wed, 29 Jul 2020 13:39:05 -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 08:35:57PM +0000, Fenghua Yu wrote: > 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?
I understood all off that. And as I user I want to run sld=fatal and bld=ratelimit to provide maximum protection, i.e. disallow split locks at all times, and ratelimit the crud SLD #AC can't catch.
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