Re: WARNING: can't access registers at asm_common_interrupt

[Jürgen Groß]

On 13.11.20 18:34, Andy Lutomirski wrote:
> On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 11/11/2020 20:15, Josh Poimboeuf wrote:
> > > On Wed, Nov 11, 2020 at 09:07:30PM +0100, Peter Zijlstra wrote:
> > > > On Wed, Nov 11, 2020 at 01:59:00PM -0600, Josh Poimboeuf wrote:
> > > > > On Wed, Nov 11, 2020 at 08:42:06PM +0100, Peter Zijlstra wrote:
> > > > > > > Would objtool have an easier time coping if this were implemented in
> > > > > > > terms of a static call?
> > > > > > I doubt it, the big problem is that there is no visibility into the
> > > > > > actual alternative text. Runtime patching fragments into static call
> > > > > > would have the exact same problem.
> > > > > >
> > > > > > Something that _might_ maybe work is trying to morph the immediate
> > > > > > fragments into an alternative. That is, instead of this:
> > > > > >
> > > > > > static inline notrace unsigned long arch_local_save_flags(void)
> > > > > > {
> > > > > >     return PVOP_CALLEE0(unsigned long, irq.save_fl);
> > > > > > }
> > > > > >
> > > > > > Write it something like:
> > > > > >
> > > > > > static inline notrace unsigned long arch_local_save_flags(void)
> > > > > > {
> > > > > >     PVOP_CALL_ARGS;
> > > > > >     PVOP_TEST_NULL(irq.save_fl);
> > > > > >     asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
> > > > > >                                     "PUSHF; POP _ASM_AX",
> > > > > >                                     X86_FEATURE_NATIVE)

I am wondering whether we really want a new feature (basically "not
XENPV). We could use ~X86_FEATURE_XENPV and teach apply_alternatives()
to understand negated features (yes, this limits the number of features
to 32767, but I don't think this is a real problem for quite some time).

Thoughts?


Juergen

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