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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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?

--Andy

>
>  Luis



-- 
Andy Lutomirski
AMA Capital Management, LLC

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