From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754718AbbAVWoW (ORCPT ); Thu, 22 Jan 2015 17:44:22 -0500 Received: from cantor2.suse.de ([195.135.220.15]:57911 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752581AbbAVWoQ (ORCPT ); Thu, 22 Jan 2015 17:44:16 -0500 Date: Thu, 22 Jan 2015 23:44:13 +0100 From: "Luis R. Rodriguez" To: Andrew Cooper Cc: Andy Lutomirski , David Vrabel , "Luis R. Rodriguez" , Konrad Rzeszutek Wilk , Boris Ostrovsky , "xen-devel@lists.xenproject.org" , "linux-kernel@vger.kernel.org" , X86 ML , kvm list , Borislav Petkov , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Jan Beulich Subject: Re: [RFC v3 1/2] x86/xen: add xen_is_preemptible_hypercall() Message-ID: <20150122224413.GU17887@wotan.suse.de> References: <1421893039-426-1-git-send-email-mcgrof@do-not-panic.com> <1421893039-426-2-git-send-email-mcgrof@do-not-panic.com> <20150122193003.GR17887@wotan.suse.de> <20150122210925.GT17887@wotan.suse.de> <54C16F2C.4090506@citrix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54C16F2C.4090506@citrix.com> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 22, 2015 at 09:44:12PM +0000, Andrew Cooper wrote: > On 22/01/2015 21:09, Luis R. Rodriguez wrote: > > On Thu, Jan 22, 2015 at 12:01:50PM -0800, Andy Lutomirski wrote: > >> On Thu, Jan 22, 2015 at 11:30 AM, Luis R. Rodriguez wrote: > >>> On Wed, Jan 21, 2015 at 07:07:36PM -0800, Andy Lutomirski wrote: > >>>> On Wed, Jan 21, 2015 at 6:17 PM, Luis R. Rodriguez > >>>> wrote: > >>>>> From: "Luis R. Rodriguez" > >>>>> > >>>>> On kernels with voluntary or no preemption we can run > >>>>> into situations where a hypercall issued through userspace > >>>>> will linger around as it addresses sub-operatiosn in kernel > >>>>> context (multicalls). Such operations can trigger soft lockup > >>>>> detection. > >>>>> > >>>>> We want to address a way to let the kernel voluntarily preempt > >>>>> such calls even on non preempt kernels, to address this we first > >>>>> need to distinguish which hypercalls fall under this category. > >>>>> This implements xen_is_preemptible_hypercall() which lets us do > >>>>> just that by adding a secondary hypercall page, calls made via > >>>>> the new page may be preempted. > >>>>> > >>>>> Andrew had originally submitted a version of this work [0]. > >>>>> > >>>>> [0] http://lists.xen.org/archives/html/xen-devel/2014-02/msg01056.html > >>>>> > >>>>> Based on original work by: Andrew Cooper > >>>>> > >>>>> Cc: Andy Lutomirski > >>>>> Cc: Borislav Petkov > >>>>> Cc: David Vrabel > >>>>> Cc: Thomas Gleixner > >>>>> Cc: Ingo Molnar > >>>>> Cc: "H. Peter Anvin" > >>>>> Cc: x86@kernel.org > >>>>> Cc: Steven Rostedt > >>>>> Cc: Masami Hiramatsu > >>>>> Cc: Jan Beulich > >>>>> Cc: linux-kernel@vger.kernel.org > >>>>> Signed-off-by: Luis R. Rodriguez > >>>>> --- > >>>>> arch/x86/include/asm/xen/hypercall.h | 20 ++++++++++++++++++++ > >>>>> arch/x86/xen/enlighten.c | 7 +++++++ > >>>>> arch/x86/xen/xen-head.S | 18 +++++++++++++++++- > >>>>> 3 files changed, 44 insertions(+), 1 deletion(-) > >>>>> > >>>>> diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h > >>>>> index ca08a27..221008e 100644 > >>>>> --- a/arch/x86/include/asm/xen/hypercall.h > >>>>> +++ b/arch/x86/include/asm/xen/hypercall.h > >>>>> @@ -84,6 +84,22 @@ > >>>>> > >>>>> extern struct { char _entry[32]; } hypercall_page[]; > >>>>> > >>>>> +#ifndef CONFIG_PREEMPT > >>>>> +extern struct { char _entry[32]; } preemptible_hypercall_page[]; > >>>> A comment somewhere explaining why only non-preemptible kernels have > >>>> preemptible hypercalls might be friendly to some future reader. :) > >>> Good idea, since this section is arch specific, I'll instead add a blurb > >>> explaining this on the upcall. > >>> > >>>>> + > >>>>> +static inline bool xen_is_preemptible_hypercall(struct pt_regs *regs) > >>>>> +{ > >>>>> + return !user_mode_vm(regs) && > >>>>> + regs->ip >= (unsigned long)preemptible_hypercall_page && > >>>>> + regs->ip < (unsigned long)preemptible_hypercall_page + PAGE_SIZE; > >>>>> +} > >>>> This makes it seem like the page is indeed one page long, but I don't > >>>> see what actually allocates a whole page for it. What am I missing? > >>> arch/x86/xen/xen-head.S > >>> > >>> .pushsection .text > >>> .balign PAGE_SIZE > >>> ENTRY(hypercall_page) > >>> > >>> #ifndef CONFIG_PREEMPT > >>> ENTRY(preemptible_hypercall_page) > >>> .skip PAGE_SIZE > >>> #endif /* CONFIG_PREEMPT */ > >>> > >>> Does that suffice to be sure? > >> This looks like hypercall_page and preemptible_hypercall_page will > >> both be page-aligned but will be the same page. Should there be > >> another .skip PAGE_SIZE in there? > > I think the trick here was since hypercall_page is already aligned, > > and we are just allocation PAGE_SIZE we are essentially pegging > > preemptible_hypercall_page right after hypercall_page. > > > > Andrew, David, can you confirm? > > Your version is different to my original one (observe the lack of > NEXT_HYPERCALL()s), I don't get what is missing they should be pretty identical. It had: --- a/arch/x86/xen/xen-head.S +++ b/arch/x86/xen/xen-head.S @@ -85,9 +85,18 @@ ENTRY(xen_pvh_early_cpu_init) .pushsection .text .balign PAGE_SIZE ENTRY(hypercall_page) + +#ifdef CONFIG_PREEMPT +# define PREEMPT_HYPERCALL_ENTRY(x) +#else +# define PREEMPT_HYPERCALL_ENTRY(x) \ + .global xen_hypercall_##x ## _p ASM_NL \ + .set preemptible_xen_hypercall_##x, xen_hypercall_##x + PAGE_SIZE ASM_NL +#endif #define NEXT_HYPERCALL(x) \ ENTRY(xen_hypercall_##x) \ - .skip 32 + .skip 32 ASM_NL \ + PREEMPT_HYPERCALL_ENTRY(x) Is that what you mean? > and I would agree that it would appear as if in your > version, hypercall_page and preemptible_hypercall_page are symbols with > the same address. > > nm should give you a quick confirmation one way or another. symtab: mcgrof@ergon ~/linux (git::hypercall-preemption-v3)$ grep hypercall_page /boot/System.map-3.19.0-rc4-00040-g9c6fb2a ffffffff81001000 T hypercall_page ffffffff81002000 T preemptible_hypercall_page This is one page apart. ffffffff81ae7b50 R __ksymtab_hypercall_page ffffffff81aea610 R __ksymtab_preemptible_hypercall_page ffffffff81af86c0 r __kcrctab_hypercall_page ffffffff81af9c20 r __kcrctab_preemptible_hypercall_page ffffffff81afbaa8 r __kstrtab_preemptible_hypercall_page ffffffff81afbac3 r __kstrtab_hypercall_page Luis