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 <mcgrof@xxxxxxxx> 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 > >>>> <mcgrof@xxxxxxxxxxxxxxxx> wrote: > >>>>> From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx> > >>>>> > >>>>> 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 <andrew.cooper3@xxxxxxxxxx> > >>>>> > >>>>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > >>>>> Cc: Borislav Petkov <bp@xxxxxxx> > >>>>> Cc: David Vrabel <david.vrabel@xxxxxxxxxx> > >>>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > >>>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx> > >>>>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > >>>>> Cc: x86@xxxxxxxxxx > >>>>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > >>>>> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> > >>>>> Cc: Jan Beulich <JBeulich@xxxxxxxx> > >>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx > >>>>> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx> > >>>>> --- > >>>>> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
