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 82D00C433FE for ; Fri, 18 Mar 2022 17:28:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239618AbiCRRaG (ORCPT ); Fri, 18 Mar 2022 13:30:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47142 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234589AbiCRRaE (ORCPT ); Fri, 18 Mar 2022 13:30:04 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9C90539693; Fri, 18 Mar 2022 10:28:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=vga+VUB1xbVQ2AIXdGqGRfNajpKT5maC9LEyJ/DlmKc=; b=oPDtOvDxgIw9zDrdY2X9mkugTv YGGbCJ2ibwPqRKTF32K9yDOEklcdjC/tbot9NOJWW9Hvo8rQEb7cKeRA0kvqCueg2vdmakrLODL7w SKshlo5SYXLSWI1m5ccGBdFeYVMr314zkrIUjOQBNS4PXDbMB+pry3Tnw6O+8E47PSoGUxffzMdRG VtgkmpU79vsc6vbATipFIEOTJp2uAa5m+YN8zvNukq4Dgcoyi9W66ZvDVpUkhodw1fusuLWZ5X3rH lEPmDufNdsREUcoeIbXWj+39n2X4KlxZYihNCW9K1uAW4f2jQtIYpwKDMs5EF8VhHwgzpZB3eV26r SWI0I6Mw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nVGP1-0088sh-Jk; Fri, 18 Mar 2022 17:28:39 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id DD1A698841D; Fri, 18 Mar 2022 18:28:37 +0100 (CET) Date: Fri, 18 Mar 2022 18:28:37 +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: <20220318172837.GQ8939@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1dc56110-5f1b-6140-937c-bf4a28ddbe87@redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Mar 18, 2022 at 05:29:20PM +0100, Paolo Bonzini wrote: > On 3/17/22 18:43, Maxim Levitsky wrote: > > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h > > index 20f64e07e359..3388072b2e3b 100644 > > --- a/arch/x86/include/asm/kvm-x86-ops.h > > +++ b/arch/x86/include/asm/kvm-x86-ops.h > > @@ -88,7 +88,7 @@ KVM_X86_OP(deliver_interrupt) > > KVM_X86_OP_OPTIONAL(sync_pir_to_irr) > > KVM_X86_OP_OPTIONAL_RET0(set_tss_addr) > > KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr) > > -KVM_X86_OP_OPTIONAL_RET0(get_mt_mask) > > +KVM_X86_OP(get_mt_mask) > > KVM_X86_OP(load_mmu_pgd) > > KVM_X86_OP(has_wbinvd_exit) > > KVM_X86_OP(get_l2_tsc_offset) > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > > index a09b4f1a18f6..0c09292b0611 100644 > > --- a/arch/x86/kvm/svm/svm.c > > +++ b/arch/x86/kvm/svm/svm.c > > @@ -4057,6 +4057,11 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index) > > return true; > > } > > +static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio) > > +{ > > + return 0; > > +} > > + > > static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) > > { > > struct vcpu_svm *svm = to_svm(vcpu); > > @@ -4718,6 +4723,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = { > > .check_apicv_inhibit_reasons = avic_check_apicv_inhibit_reasons, > > .apicv_post_state_restore = avic_apicv_post_state_restore, > > + .get_mt_mask = svm_get_mt_mask, > > .get_exit_info = svm_get_exit_info, > > .vcpu_after_set_cpuid = svm_vcpu_after_set_cpuid, > > Thanks, I'll send it as a complete patch. Please reply there with your > Signed-off-by. Yeah, ret0 should only be used with up-to 'long' return values. So ACK on that patch. > 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 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.