linux-kernel - Re: [RFC PATCH 3/7] x86/entry: Implement atomic-IST-entry

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Date:   Fri, 7 Apr 2023 00:07:46 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Cfir Cohen <cfir@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Kaplan <David.Kaplan@....com>,
        David Rientjes <rientjes@...gle.com>,
        Dirk Hohndel <dirkhh@...are.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Jiri Slaby <jslaby@...e.cz>, Joerg Roedel <joro@...tes.org>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Stunes <mstunes@...are.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tony Luck <tony.luck@...el.com>, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 3/7] x86/entry: Implement atomic-IST-entry

On 06/04/2023 10:58 pm, Peter Zijlstra wrote:
> On Mon, Apr 03, 2023 at 10:06:01PM +0800, Lai Jiangshan wrote:
>> +/* Replicate the interrupted atomic-IST-entry's CLEAR_REGS macro. */
>> +static __always_inline void replicate_clear_regs(struct pt_regs *target)
>> +{
>> +	target->di  = 0;
>> +	target->si  = 0;
>> +	target->dx  = 0;
>> +	target->cx  = 0;
>> +	target->ax  = 0;
>> +	target->r8  = 0;
>> +	target->r9  = 0;
>> +	target->r10 = 0;
>> +	target->r11 = 0;
>> +	target->bx  = 0;
>> +	target->bp  = 0;
>> +	target->r12 = 0;
>> +	target->r13 = 0;
>> +	target->r14 = 0;
>> +	target->r15 = 0;
>> +}
> I think there's compilers smart enough to see through your attempts at
> avoiding mem{set,cpy}() there

Indeed.  It took a little bit of convincing (needed 4 extra registers to
zero), but https://godbolt.org/z/7rvb8db66

Including Peter's other observation about speculation safety, you
basically can't have any C at all before passing IBRS/UNRET/whatever
else comes along in the future.

Otherwise, the compiler will make you wish you'd written it in asm the
first time around.

~Andrew