Re: [RFC v4 1/2] x86/xen: add xen_is_preemptible_hypercall()

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

On Fri, Jan 23, 2015 at 11:30:16AM +0000, David Vrabel wrote:
> On 23/01/15 00:29, Luis R. Rodriguez 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.
> [...]
> > --- 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[];
> > +
> > +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;
> 
> I think you can optimize this to:
> 
>     return (regs->ip >> PAGE_SHIFT) == preemptible_hypercall_pfn

I take it you meant preemptible_hypercall_page ?

> 	&& !user_mode_vm(regs);

If so I don't see how this can work as an identical replacement.
Consider a PAGE_SIZE is 16, so PAGE_SHIFT would be 4, and lets
say we are checking for byte 2 which should be in the page:

; 0b0010 >>4
	0

Can you elaborate more on this, or can we perhaps leave such
optimization as an evolution to avoid regressions if you are
not 100% certain?

  Luis