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? 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
