linux-kernel - Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask

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Date:   Thu, 1 Sep 2022 09:37:42 +0200
From:   Gabriel Paubert <paubert@...m.es>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Zhouyi Zhou <zhouzhouyi@...il.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and
 irq_soft_mask_return() with sanitizer

On Thu, Sep 01, 2022 at 05:22:32AM +0000, Christophe Leroy wrote:
> 
> 
> Le 01/09/2022 à 00:45, Segher Boessenkool a écrit :
> > Hi!
> > 
> > On Tue, Aug 30, 2022 at 09:10:02AM +0000, Christophe Leroy wrote:
> >> Le 30/08/2022 à 11:01, Nicholas Piggin a écrit :
> >>> On Tue Aug 30, 2022 at 3:24 PM AEST, Christophe Leroy wrote:
> >>>>> This is still slightly concerning to me. Is there any guarantee that the
> >>>>> compiler would not use a different sequence for the address here?
> >>>>>
> >>>>> Maybe explicit r13 is required.
> >>>>>
> >>>>
> >>>> local_paca is defined as:
> >>>>
> >>>> 	register struct paca_struct *local_paca asm("r13");
> > 
> > And this is in global scope, making it a global register variable.
> > 
> >>>> Why would the compiler use another register ?
> >>>
> >>> Hopefully it doesn't. Is it guaranteed that it won't?
> > 
> > Yes, this is guaranteed.
> > 
> > For a local register variable this is guaranteed only for operands to an
> > extended inline asm; any other access to the variable does not have to
> > put it in the specified register.
> > 
> > But this is a global register variable.  The only thing that would make
> > this crash and burn is if *any* translation unit did not see this
> > declaration: it could then use r13 (if that was allowed by the ABI in
> > effect, heh).
> > 
> >>> I'm sure Segher will be delighted with the creative asm in __do_IRQ
> >>> and call_do_irq :) *Grabs popcorn*
> > 
> > All that %% is blinding, yes.
> > 
> > Inline tabs are bad taste.
> > 
> > Operand names instead of numbers are great for obfuscation, and nothing
> > else -- unless you have more than four or five operands, in which case
> > you have bigger problems already.
> > 
> > Oh, this function is a good example of proper use of local register asm,
> > btw.
> > 
> > Comments like "// Inputs" are just harmful.  As is the "creative"
> > indentation here.  Both harm readability and do not help understanding
> > in any other way either.
> > 
> > The thing about inline asm is the smallest details change meaning of the
> > whole, it is a very harsh environment, you are programming both in C and
> > directly assembler code as well, and things have to be valid for both,
> > although on the other hand there is almost no error checking.  Keeping
> > it small, simple, readable is paramount.
> > 
> > The rules for using inline asm:
> > 
> > 0) Do no use inline asm.
> > 1) Use extended asm, unless you know all differences with basic asm, and
> >     you know you want that.  And if you answer "yes I do" to the latter,
> >     you answered wrong to the former.
> > 2) Do not use toplevel asm.
> > 3) Do no use inline asm.
> > 4) Do no use inline asm.
> > 5) Do no use inline asm.
> > 
> > Inline asm is a very powerful escape hatch.  Like all emergency exits,
> > you should not use them if you do not need them!  :-)
> > 
> > But, you are talking about the function calling and the frame change I
> > bet :-)  Both of these are only okay because everything is back as it
> > was when this (single!) asm is done, and the state created is perfectly
> > fine (this is very dependent on exact ABI used, etc.)
> > 
> > I would have used real assembler code here (in a .s file).  But there
> > probably are reasons to do things this way, performance probably?
> 
> We changed it to inline asm in order to ... inline it in the caller.

And there is a single caller it seems. Typically GCC inlines function
with a single call site, but it may be confused by asm statements.

> 
> I also find that those operand names make it awull more difficult to 
> read that traditional numbering. I really dislike that new trend.
> And same with those // comments, better use meaningfull C variable names.

Agree, but there is one thing which escapes me: why is r3 listed in the
outputs section (actually as a read write operand with the "+"
constraint modifier) but is not used after the asm which is the last
statement of function returning void?

Do I miss something?

	Gabriel