Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)

[Andrew Cooper]

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