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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@suse.com> 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@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?
>> 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