Messages in this thread | | | Date | Wed, 11 Nov 2020 12:13:28 -0600 | From | Josh Poimboeuf <> | Subject | Re: WARNING: can't access registers at asm_common_interrupt |
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On Wed, Nov 11, 2020 at 06:47:36PM +0100, Peter Zijlstra wrote: > This is PARAVIRT_XXL only, which is a Xen special. My preference, as > always, is to kill it... Sadly the Xen people have a different opinion.
That would be soooo nice... then we could get rid of paravirt patching altogether and replace it with static calls.
> > Objtool doesn't know about the pushf/pop paravirt patch, so ORC gets > > confused by the changed stack layout. > > > > I'm thinking we either need to teach objtool how to deal with > > save_fl/restore_fl patches, or we need to just get rid of those nasty > > patches somehow. Peter, any thoughts? > > Don't use Xen? ;-) > > So with PARAVIRT_XXL the compiler will emit something like: > > "CALL *pvops.save_fl" > > Which we then overwrite at runtime with "pushf; pop %[re]ax" and a few > NOPs. > > Now, objtool understands alternatives, and ensures they have the same > stack layout, it has no chance in hell of understanding this, simply > because paravirt_patch.c is magic. > > I don't have any immediate clever ideas, but let me ponder it a wee bit. > > .... > > Something really disguisting we could do is recognise the indirect call > offset and emit an extra ORC entry for RIP+1. So the cases are: > > CALL *pv_ops.save_fl -- 7 bytes IIRC > CALL $imm; -- 5 bytes > PUSHF; POP %[RE]AX -- 2 bytes > > so the RIP+1 (the POP insn) will only ever exist in this case. The > indirect and direct call cases would never land on that IP.
I had a similar idea, and a bit of deja vu - we may have talked about this before. At least I know we talked about doing something similar for alternatives which muck with the stack.
> > It looks like 044d0d6de9f5 ("lockdep: Only trace IRQ edges") is making > > the problem more likely, by adding the irqs_disabled() check for every > > local_irq_disable(). > > > > Also - Peter, Nicholas - is that irqs_disabled() check really necessary > > in local_irq_disable()? Presumably irqs would typically be be enabled > > before calling it? > > Yeah, so it's all a giant can of worms that; also see: > > https://lkml.kernel.org/r/20200821084738.508092956@infradead.org > > The basic idea is to only trace edges, ie. when the hardware state > actually changes. Sadly this means doing a pushf/pop before the cli. > Ideally CLI would store the old IF in CF or something like that, but > alas.
Right, that makes sense for save/restore, but is the disabled check really needed for local_irq_disable()? Wouldn't that always be an edge?
And anyway I don't see a similar check for local_irq_enable().
-- Josh
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