linux-kernel - Re: [patch V4 part 3 09/29] x86/entry/32: Provide macro to emit IDT entry stubs

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Message-ID: <1225010168.20900.1589463536204.JavaMail.zimbra@efficios.com>
Date:   Thu, 14 May 2020 09:38:56 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, paulmck <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>, rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 3 09/29] x86/entry/32: Provide macro to emit IDT
 entry stubs

----- On May 14, 2020, at 12:31 AM, Andy Lutomirski luto@...nel.org wrote:

> On Wed, May 13, 2020 at 6:44 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> ----- On May 5, 2020, at 9:44 AM, Thomas Gleixner tglx@...utronix.de wrote:
>>
>> [...]
>>
>> > +.macro idtentry vector asmsym cfunc has_error_code:req sane=0
>> > +SYM_CODE_START(\asmsym)
>> > +     ASM_CLAC
>> > +     cld
>>
>> Looking at the various interrupt and trap entry points for 32 and 64-bit
>> x86, I notice a lack of consistency in use of the following instruction
>> sequence at the asm entry point:
>>
>> - ASM_CLAC,
>> - cld (clear direction flag).
>>
>> Are they always needed, or only for interrupt handlers ?
> 
> They're needed for all entries except SYSCALL, but they're hidden
> inside helpers in many cases.

Indeed, on x86-32 the macro SAVE_ALL contains cld. That architecture
appears to be OK.

What I am concerned about is the idtentry, idtentry_mce_db,
and idtentry_df macros introduced in entry_64.S by this series.
Those are supposed to be technically equivalent to the prior
code, which indeed has the ASM_CLAC but no "cld".

So maybe the cld happens to be hidden elsewhere, but I'm clearly
missing it ? Or is it not needed for some reason ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com