linux-kernel - Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync

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Message-ID: <20200805170717.GB26661@ranerica-svr.sc.intel.com>
Date:   Wed, 5 Aug 2020 10:07:17 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     hpa@...or.com, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Cathy Zhang <cathy.zhang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Kyung Min Park <kyung.min.park@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-edac@...r.kernel.org
Subject: Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync_core() when available

On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote:
> On Tue, Aug 04, 2020 at 09:58:25PM -0700, hpa@...or.com wrote:
> > Because why use an alternative to jump over one instruction?
> >
> > I personally would prefer to have the IRET put out of line
> 
> Can't yet - SERIALIZE CPUs are a minority at the moment.
> 
> > and have the call/jmp replaced by SERIALIZE inline.
> 
> Well, we could do:
> 
> 	alternative_io("... IRET bunch", __ASM_SERIALIZE, X86_FEATURE_SERIALIZE, ...);
> 
> and avoid all kinds of jumping. Alternatives get padded so there
> would be a couple of NOPs following when SERIALIZE gets patched in
> but it shouldn't be a problem. I guess one needs to look at what gcc
> generates...

But the IRET-TO-SELF code has instruction which modify the stack. This
would violate stack invariance in alternatives as enforced in commit
7117f16bf460 ("objtool: Fix ORC vs alternatives"). As a result, objtool
gives warnings as follows:

arch/x86/kernel/alternative.o: warning: objtool: do_sync_core()+0xe:
alternative modifies stack

Perhaps in this specific case it does not matter as the changes in the
stack will be undone by IRET. However, using alternative_io would require
adding the macro STACK_FRAME_NON_STANDARD to functions using sync_core().
IMHO, it wouldn't look good.

So maybe the best approach is to implement as you suggested using
static_cpu_has()?

Thanks and BR,
Ricardo