Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932241AbcLBTev (ORCPT ); Fri, 2 Dec 2016 14:34:51 -0500 Received: from mail-vk0-f41.google.com ([209.85.213.41]:33141 "EHLO mail-vk0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751957AbcLBTet (ORCPT ); Fri, 2 Dec 2016 14:34:49 -0500 MIME-Version: 1.0 In-Reply-To: <402ae08c-22d3-bec7-6649-26632c941a29@oracle.com> References: <0a21157c2233ba7d0781bbf07866b3f2d7e7c25d.1480638597.git.luto@kernel.org> <402ae08c-22d3-bec7-6649-26632c941a29@oracle.com> From: Andy Lutomirski Date: Fri, 2 Dec 2016 11:34:26 -0800 Message-ID: Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation To: Boris Ostrovsky Cc: Xen-devel List , Juergen Gross , Borislav Petkov , Matthew Whitehead , One Thousand Gnomes , Henrique de Moraes Holschuh , Andrew Cooper , Brian Gerst , "linux-kernel@vger.kernel.org" , X86 ML , Peter Zijlstra 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: 2566 Lines: 66 On Dec 2, 2016 10:48 AM, "Boris Ostrovsky" wrote: > > On 12/02/2016 06:44 AM, Andrew Cooper wrote: > > On 02/12/16 00:35, Andy Lutomirski wrote: > >> On Xen PV, CPUID is likely to trap, and Xen hypercalls aren't > >> guaranteed to serialize. (Even CPUID isn't *really* guaranteed to > >> serialize on Xen PV, but, in practice, any trap it generates will > >> serialize.) > > Well, Xen will enabled CPUID Faulting wherever it can, which is > > realistically all IvyBridge hardware and newer. > > > > All hypercalls are a privilege change to cpl0. I'd hope this condition > > is serialising, but I can't actually find any documentation proving or > > disproving this. > > > >> On my laptop, CPUID(eax=1, ecx=0) is ~83ns and IRET-to-self is > >> ~110ns. But Xen PV will trap CPUID if possible, so IRET-to-self > >> should end up being a nice speedup. > >> > >> Cc: Andrew Cooper > >> Signed-off-by: Andy Lutomirski > > CC'ing xen-devel and the Xen maintainers in Linux. > > > > As this is the only email from this series in my inbox, I will say this > > here, but it should really be against patch 6. > > > > A write to %cr2 is apparently (http://sandpile.org/x86/coherent.htm) not > > serialising on the 486, but I don't have a manual to hand to check. > > > > ~Andrew > > > >> --- > >> arch/x86/xen/enlighten.c | 35 +++++++++++++++++++++++++++++++++++ > >> 1 file changed, 35 insertions(+) > >> > >> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c > >> index bdd855685403..1f765b41eee7 100644 > >> --- a/arch/x86/xen/enlighten.c > >> +++ b/arch/x86/xen/enlighten.c > >> @@ -311,6 +311,39 @@ static __read_mostly unsigned int cpuid_leaf1_ecx_set_mask; > >> static __read_mostly unsigned int cpuid_leaf5_ecx_val; > >> static __read_mostly unsigned int cpuid_leaf5_edx_val; > >> > >> +static void xen_sync_core(void) > >> +{ > >> + register void *__sp asm(_ASM_SP); > >> + > >> +#ifdef CONFIG_X86_32 > >> + asm volatile ( > >> + "pushl %%ss\n\t" > >> + "pushl %%esp\n\t" > >> + "addl $4, (%%esp)\n\t" > >> + "pushfl\n\t" > >> + "pushl %%cs\n\t" > >> + "pushl $1f\n\t" > >> + "iret\n\t" > >> + "1:" > >> + : "+r" (__sp) : : "cc"); > > This breaks 32-bit PV guests. > > Why are we pushing %ss? We are not changing privilege levels so why not > just flags, cs and eip (which, incidentally, does work)? > Doh! I carefully tested 64-bit on Xen and 32-bit in user mode.