Messages in this thread | | | Subject | Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V) | From | Andrew Cooper <> | Date | Wed, 3 Jun 2020 20:18:44 +0100 |
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On 22/05/2020 22:17, Peter Zijlstra wrote: > On Fri, May 22, 2020 at 08:20:15AM +0100, Andrew Cooper wrote: >> Apologies for opening a related can of worms. >> >> The new debug_enter() has propagated a pre-existing issue forward, >> ultimately caused by bad advice in the SDM. >> >> Because the RTM status bit in DR6 has inverted polarity, writing DR6 to >> 0 causes RTM to appear asserted to any logic which cares, despite RTM >> debugging not being enabled. The same is true in principle for what is >> handed to userspace via u_debugreg[DR_STATUS]. >> >> On the subject of DR6, the SDM now reads: >> >> "Certain debug exceptions may clear bits 0-3. The remaining contents of >> the DR6 register are never cleared by the processor. To avoid confusion >> in identifying debug exceptions, debug handlers should clear the >> register (except bit 16, which they should set) before returning to the >> interrupted task." > *URGH* > >> First of all, that should read "are never de-asserted by the processor" >> rather than "cleared", but the advice has still failed to learn from its >> first mistake. The forward-compatible way to fix this is to set >> DR6_DEFAULT (0xffff0ff0) which also covers future inverted polarity bits. >> >> As for what to do about userspace, that is harder. One approach is to >> express everything in terms of positive polarity (i.e. pass on dr6 ^ >> DR6_DEFAULT), so DR6_RTM only appears set when RTM debugging is >> enabled. This approach is already taken with the VMCS PENDING_DBG >> field, so there is at least previous form. >> >> I realise that "do nothing" might be acceptable at this point, given the >> lack of support for RTM debugging. > This! I'm thinking "do nothing" is, at this moment, the right thing to > do. If/when someone goes and tries to make RTM debugging work, they get > to figure out how to deal with this mess.
Well that didn't last long...
The new ISE (rev 39, published today) introduces BUS LOCK DEBUG EXCEPTION which is now a second inverted polarity sticky bit (bit 11) in %dr6.
This one is liable to get more traction than RTM debugging, so something probably does want fixing in the #DB handler.
~Andrew
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