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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v2] x86/cpu: Use SERIALIZE in sync_core() when available
Date:   Wed, 5 Aug 2020 13:30:32 -0700
Message-Id: <AC4AC665-06C3-404A-8245-BA4F1F4C4961@amacapital.net>
References: <20200805191126.GA27509@ranerica-svr.sc.intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Cathy Zhang <cathy.zhang@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Kyung Min Park <kyung.min.park@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-edac <linux-edac@vger.kernel.org>
In-Reply-To: <20200805191126.GA27509@ranerica-svr.sc.intel.com>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Aug 5, 2020, at 12:11 PM, Ricardo Neri <ricardo.neri-calderon@linux.int=
el.com> wrote:
>=20
> =EF=BB=BFOn Wed, Aug 05, 2020 at 11:28:31AM -0700, Andy Lutomirski wrote:
>>> On Wed, Aug 5, 2020 at 10:07 AM Ricardo Neri
>>> <ricardo.neri-calderon@linux.intel.com> wrote:
>>>=20
>>> On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote:
>>>> On Tue, Aug 04, 2020 at 09:58:25PM -0700, hpa@zytor.com wrote:
>>>>> Because why use an alternative to jump over one instruction?
>>>>>=20
>>>>> I personally would prefer to have the IRET put out of line
>>>>=20
>>>> Can't yet - SERIALIZE CPUs are a minority at the moment.
>>>>=20
>>>>> and have the call/jmp replaced by SERIALIZE inline.
>>>>=20
>>>> Well, we could do:
>>>>=20
>>>>      alternative_io("... IRET bunch", __ASM_SERIALIZE, X86_FEATURE_SERI=
ALIZE, ...);
>>>>=20
>>>> 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...
>>>=20
>>> 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:
>>>=20
>>> arch/x86/kernel/alternative.o: warning: objtool: do_sync_core()+0xe:
>>> alternative modifies stack
>>>=20
>>> 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 requir=
e
>>> adding the macro STACK_FRAME_NON_STANDARD to functions using sync_core()=
.
>>> IMHO, it wouldn't look good.
>>>=20
>>> So maybe the best approach is to implement as you suggested using
>>> static_cpu_has()?
>>=20
>> I agree.  Let's keep it simple.
>>=20
>> Honestly, I think the right solution is to have iret_to_self() in
>> actual asm and invoke it from C as needed.=20
>=20
> Do you mean anything different from what we have already [1]? If I
> understand your comment correctly, we have exactly that: an
> iret_to_self() asm implementation invoked from C.

I meant asm as in a .S file. But the code we have is fine for this purpose, a=
t least for now.

>=20
> Thanks and BR,
> Ricardo
>=20
> [1]. https://lore.kernel.org/lkml/20200727043132.15082-4-ricardo.neri-cald=
eron@linux.intel.com/
>=20
> Thanks and BR,
> Ricardo