linux-kernel - Re: WARNING: can't access registers at asm_common

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Message-ID: <20201118082254.GH3121378@hirez.programming.kicks-ass.net>
Date:   Wed, 18 Nov 2020 09:22:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        X86 ML <x86@...nel.org>
Subject: Re: WARNING: can't access registers at asm_common_interrupt

On Wed, Nov 18, 2020 at 07:47:36AM +0100, Jürgen Groß wrote:
> On 16.11.20 14:04, Peter Zijlstra wrote:
> > On Mon, Nov 16, 2020 at 12:56:32PM +0100, Jürgen Groß wrote:
> > > > > > > > > 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?
> > 
> > I went with the simple thing for now... If we ever want/need another
> > negative alternative I suppose we can do as you suggest...
> > 
> > I was still poking at objtool to actually dtrt though..
> 
> I'd like to include this part in my series (with a different solution
> for the restore_fl part, as suggested by Andy before).
> 
> Peter, are you fine with me taking your patch and adding your SoB?

Sure, note that I only compile tested it, as it was my test-bed for
playing with objtool (which I still haven't managed to get back to and
finish :/).

So if it actually _works_ feel free to take it, otherwise you can have
whatever bits and pieces remain after you've butchered it to do as you
want.