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

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

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

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