Messages in this thread Patch in this message | | | Date | Fri, 23 Jan 2015 19:57:11 +0100 | From | "Luis R. Rodriguez" <> | Subject | Re: [Xen-devel] [RFC v4 1/2] x86/xen: add xen_is_preemptible_hypercall() |
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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
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