From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3545AC433F5 for ; Fri, 18 Mar 2022 18:02:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239890AbiCRSDv (ORCPT ); Fri, 18 Mar 2022 14:03:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60114 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231842AbiCRSDt (ORCPT ); Fri, 18 Mar 2022 14:03:49 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7F7782E8CFF; Fri, 18 Mar 2022 11:02:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=0Yy9CpCZpGFGgUuEZDwIZX23n2xP8D/MS6pjjuaf+JM=; b=NYskq7i5ZtPpWLlAkMYkR/LoTK zRjWAbokWOXzl+7c1BzPUVaFL0Kh45L4+/WqPH7DtoZGuxLCCXwrfyoVNBCkKWGd6OFg2uSjBLyXx yOdh8T3UdJ8+vnUKJFDPCdIlVzv04ktUVp0xXLXS5GbiaYyvI7DRl2MqBK382YHPb7qtHuDvYZAWO UqlGqvaiR+qnja7Oc2OzN/Pm2IfevxZp/ZV7/i3aB1+GVeUG+zjhWvSonVEyBIuzaHiqhmkP8PQu9 obfxjy+UU6kK8TCAJjth+hCpY28s9KscdAENjEoyvQpY3AcukqNVJZe5vs8oQnv4kV3VTVGTQi4HL jux9TOAg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nVGvi-002GhT-1v; Fri, 18 Mar 2022 18:02:26 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id BB20598841D; Fri, 18 Mar 2022 19:02:25 +0100 (CET) Date: Fri, 18 Mar 2022 19:02:25 +0100 From: Peter Zijlstra To: Paolo Bonzini Cc: Maxim Levitsky , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, seanjc@google.com Subject: Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls Message-ID: <20220318180225.GF14330@worktop.programming.kicks-ass.net> References: <20220217180831.288210-1-pbonzini@redhat.com> <20220217180831.288210-7-pbonzini@redhat.com> <3bbe3f8717cdf122f909a48e117dab6c09d8e0c8.camel@redhat.com> <1dc56110-5f1b-6140-937c-bf4a28ddbe87@redhat.com> <20220318172837.GQ8939@worktop.programming.kicks-ass.net> <20220318174732.GE14330@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220318174732.GE14330@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Mar 18, 2022 at 06:47:32PM +0100, Peter Zijlstra wrote: > On Fri, Mar 18, 2022 at 06:28:37PM +0100, Peter Zijlstra wrote: > > > Related to this, I don't see anything in arch/x86/kernel/static_call.c that > > > limits this code to x86-64: > > > > > > if (func == &__static_call_return0) { > > > emulate = code; > > > code = &xor5rax; > > > } > > > > > > > > > On 32-bit, it will be patched as "dec ax; xor eax, eax" or something like > > > that. Fortunately it doesn't corrupt any callee-save register but it is not > > > just a bit funky, it's also not a single instruction. > > > > Urggghh.. that's fairly yuck. So there's two options I suppose: > > > > 0x66, 0x66, 0x66, 0x31, 0xc0 > > Argh, that turns into: xorw %ax, %ax. > > Let me see if there's another option. Amazingly: 0x2e, 0x2e, 0x2e, 0x31, 0xc0 seems to actually work.. I've build and ran and decoded the below on 32bit and 64bit (arguably on the same 64bit host). --- #include long zero(void) { long z = -1L; asm (".byte 0x2e, 0x2e, 0x2e, 0x31, 0xc0" : "=a" (z) ); return z; } void main(void) { printf("%ld\n", zero()); }