On 08/08/17 22:09, Andy Lutomirski wrote:
> On Tue, Aug 8, 2017 at 12:13 PM, Josh Poimboeuf <jpoim...@redhat.com> 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 <jpoim...@redhat.com> 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:
>>>> pop %rax
>>>> The problem is when an interrupt hits after the push:
>>>> --- 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
>>> 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.
>> 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",
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. 
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-de...@lists.xenproject.org (with PVH support for guests nearly
complete your chances might be >0 to succeed).