| Date | Fri, 22 May 2020 23:17:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V) |
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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.
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