Re: [Xen-devel] [PATCH 2/2] x86/hvm: honor guest's option when updating secondary system time for guest

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, July 23, 2014 8:19 PM
> To: Wu, Feng
> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx;
> keir@xxxxxxx; tim@xxxxxxx
> Subject: Re: [PATCH 2/2] x86/hvm: honor guest's option when updating
> secondary system time for guest
> 
> >>> On 08.07.14 at 01:18, <feng.wu@xxxxxxxxx> wrote:
> > --- a/xen/include/public/vcpu.h
> > +++ b/xen/include/public/vcpu.h
> > @@ -227,6 +227,16 @@ struct vcpu_register_time_memory_area {
> >  typedef struct vcpu_register_time_memory_area
> > vcpu_register_time_memory_area_t;
> >  DEFINE_XEN_GUEST_HANDLE(vcpu_register_time_memory_area_t);
> >
> > +/*
> > + * Flags to tell Xen whether we need to do the SMAP check when updating
> > + * the secondary copy of the vcpu time when SMAP is enabled. Since the
> > + * memory location for the secondary copy of the vcpu time may be mapped
> > + * into userspace by guests intendedly, we let the guest to determine
> > + * whether the check is needed. The default behavior of hypevisor is
> > + * not doing the check.
> > + */
> > +#define VCPUOP_enable_smap_check_vcpu_time_memory_area   14
> 
> I think the new op to be VCPUOP_register_vcpu_time_memory_area_smap,
> identical to VCPUOP_register_vcpu_time_memory_area apart from also
> setting the flag, would be more natural. But considering what I just wrote
> in the reply to Tim I guess we can expect a nun-user mapping to be
> presented here anyway, i.e. we wouldn't need to new operation at all.

Do you mean since the user-paging is r/o, guest will pass a r/w kernel page to
Xen for updating the system time. So we don't need to do the SMAP check
in this case?

Thanks,
Feng
> 
> Jan


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