Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933079AbbKRUWS (ORCPT ); Wed, 18 Nov 2015 15:22:18 -0500 Received: from mail-ob0-f176.google.com ([209.85.214.176]:34930 "EHLO mail-ob0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932291AbbKRUWR (ORCPT ); Wed, 18 Nov 2015 15:22:17 -0500 MIME-Version: 1.0 In-Reply-To: <1447877179-2775-2-git-send-email-boris.ostrovsky@oracle.com> References: <1447877179-2775-1-git-send-email-boris.ostrovsky@oracle.com> <1447877179-2775-2-git-send-email-boris.ostrovsky@oracle.com> From: Andy Lutomirski Date: Wed, 18 Nov 2015 12:21:56 -0800 Message-ID: Subject: Re: [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests To: Boris Ostrovsky Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Vrabel , Konrad Rzeszutek Wilk , Andrew Lutomirski , Borislav Petkov , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xenproject.org" , Linux Virtualization Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2140 Lines: 56 On Wed, Nov 18, 2015 at 12:06 PM, Boris Ostrovsky wrote: > After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c > ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack > frame that is passed to xen_sysexit is no longer a "standard" one (i.e. > it's not pt_regs). > > Since we end up calling xen_iret from xen_sysexit we don't need to fix > up the stack and instead follow entry_SYSENTER_32's IRET path directly > to xen_iret. > > We can do the same thing for compat mode even though stack does not need > to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in > the subsequent patch) Looks generally quite nice. Minor comments below: > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -308,7 +308,8 @@ sysenter_past_esp: > > movl %esp, %eax > call do_fast_syscall_32 > - testl %eax, %eax > + /* XEN PV guests always use IRET path */ > + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV > jz .Lsyscall_32_done Could we make this a little less subtle: ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp .Lsyscasll_32_done", X86_FEATURE_XENPV Borislav, what do you think? Ditto for the others. > diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h > index e4f8010..0e4fe5b 100644 > --- a/arch/x86/include/asm/cpufeature.h > +++ b/arch/x86/include/asm/cpufeature.h > @@ -216,6 +216,7 @@ > #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */ > #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */ > #define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */ > +#define X86_FEATURE_XENPV ( 8*32+16) /* Xen paravirtual guest */ > This bit is highly magical and I think we need Borislav's ack. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/