Re: [PATCH v4 1/2] x86/unwind: add ORC unwinder

[Juergen Gross]

On 08/08/17 22:09, Andy Lutomirski wrote:
> On Tue, Aug 8, 2017 at 12:13 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> On Tue, Aug 08, 2017 at 12:03:51PM -0700, Linus Torvalds wrote:
>>> On Tue, Aug 8, 2017 at 11:58 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>>>>
>>>> Take for example the lock_is_held_type() function.  In vmlinux, it has
>>>> the following instruction:
>>>>
>>>>   callq *0xffffffff85a94880 (pv_irq_ops.save_fl)
>>>>
>>>> At runtime, that instruction is patched and replaced with a fast inline
>>>> version of arch_local_save_flags() which eliminates the call:
>>>>
>>>>   pushfq
>>>>   pop %rax
>>>>
>>>> The problem is when an interrupt hits after the push:
>>>>
>>>>   pushfq
>>>>   --- irq ---
>>>>   pop %rax
>>>
>>> That should actually be something easily fixable, for an odd reason:
>>> the instruction boundaries are different.
>>>
>>>> I'm not sure what the solution should be.  It will probably need to be
>>>> one of the following:
>>>>
>>>>   a) either don't allow runtime "alternative" patches to mess with the
>>>>      stack pointer (objtool could enforce this); or
>>>>
>>>>   b) come up with some way to register such patches with the ORC
>>>>      unwinder at runtime.
>>>
>>> c) just add ORC data for the alternative statically and _unconditionally_.
>>>
>>> No runtime registration. Just an unconditional entry for the
>>> particular IP that comes after the "pushfq". It cannot match the
>>> "callq" instruction, since it would be in the middle of that
>>> instruction.
>>>
>>> Basically, just do a "union" of the ORC data for all the alternatives.
>>>
>>> Now, objtool should still verify that the instruction pointers for
>>> alternatives are unique - or that they share the same ORC unwinder
>>> information if they are not.
>>>
>>> But in cases like this, when the instruction boundaires are different,
>>> things should "just work", with no need for any special cases.
>>>
>>> Hmm?
>>
>> Yeah, that might work.  Objtool already knows about alternatives, so it
>> might not be too hard.  I'll try it.
> 
> But this one's not an actual alternative, right?  It's a pv op.
> 
> I would advocate that we make it an alternative after all.  I frickin'
> hate the PV irq ops.  It would like roughly like this:
> 
> ALTERNATIVE "pushfq; popq %rax", "callq *pv_irq_ops.save_fl",
> X86_FEATURE_GODDAMN_PV_IRQ_OPS

You are aware that at least some of the Xen irq pvops functionality is
patched inline? Your modification would slow down pv guests quite a
bit, I guess.

> (The obvious syntax error and the naming should probably be fixed.
> Also, this needs to live in an #ifdef because it needs to build on
> kernels with pv support.  It should also properly register itself as a
> pv patch site.)
> 
> Semi-serious question: can we maybe delete lguest and 32-bit Xen PV
> support some time soon?  As far as I know, 32-bit Xen PV *hosts* are
> all EOL and have no security support, 32-bit Xen PV guest dom0 may not
> work (I've never tried, but it would certainly be nutty on a 64-bit
> hypervisor), and lguest is, um, not seriously maintained any more. [1]

Hmm, I suggested drop of lguest support about 3 months ago and got no
response. OTOH there was no objection either. :-)

Regarding 32 bit Xen PV guests: even 32 bit dom0 is supposed to work.
In case you want to drop support in Linux you might ask that question
on xen-devel@xxxxxxxxxxxxxxxxxxxx (with PVH support for guests nearly
complete your chances might be >0 to succeed).


Juergen