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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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. :)

> +
> +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?

--Andy

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