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), 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. ~Andrew -- 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
