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 318BDC433F5 for ; Fri, 18 Mar 2022 17:47:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239755AbiCRRs6 (ORCPT ); Fri, 18 Mar 2022 13:48:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57968 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236763AbiCRRs5 (ORCPT ); Fri, 18 Mar 2022 13:48:57 -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 ED18130DC7A; Fri, 18 Mar 2022 10:47:37 -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=jtXAoNk4oHdT6AchqGCksjwUQyf3YGIaQZHARzJgBPI=; b=SB0qvOOM4dSIeWvJQgJRbW9rIR ZXbNVm78f4P8tMst7oZjSVCKBlA+qJKczOrAnyuQ4uKcpm2/YAsRNwevrc9PEwEIpkuSNpW2ei8KA 70WS8JDsADQqBefxhS4BM3x96Ef5khyZhsTUkdj6jvpodQQKt+bbezGgpg0aEXvwX2/5u+4D0qBwK Le4JBou9OKvSQ+SLJ6MHVBgBsTyF1nnXdXB0u2nsQwwEfeML2bsZvPa+vaJFQ4pSDwWJN8rP+JIkY 7zgzZoY+zdIR6nAjqzVBLD7xni1CBuPTRGpSLJEdQvqX4z1t633g+MmitE19jGfQ+H54IeHyDAEQO mgmXKtDQ==; 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 1nVGhJ-002GU8-Dl; Fri, 18 Mar 2022 17:47:33 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id BF02698841D; Fri, 18 Mar 2022 18:47:32 +0100 (CET) Date: Fri, 18 Mar 2022 18:47:32 +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: <20220318174732.GE14330@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220318172837.GQ8939@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > Which is a tripple prefix xor %eax, %eax, which, IIRC should still clear > the whole 64bit on 64bit and *should* still not trigger the prefix > decoding penalty some frontends have (which is >3 IIRC). > > Or we can emit: > > 0xb8, 0x00, 0x00, 0x00, 0x00 > > which decodes to: mov $0x0,%eax, which is less efficient in some > front-ends since it doesn't always get picked up in register rename > stage. > >