Re: [RFC v3 1/2] x86/xen: add xen_is_preemptible_hypercall()

["Luis R. Rodriguez" <mcgrof@suse.com>]

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@do-not-panic.com> wrote:
> > From: "Luis R. Rodriguez" <mcgrof@suse.com>
> >
> > 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@citrix.com>
> >
> > Cc: Andy Lutomirski <luto@amacapital.net>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: David Vrabel <david.vrabel@citrix.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: x86@kernel.org
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> > Cc: Jan Beulich <JBeulich@suse.com>
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
> > ---
> >  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?

 Luis