> -----Original Message----- > From: Jan Beulich [mailto:jbeul...@suse.com] > Sent: Wednesday, March 9, 2016 8:37 PM > To: andrew.coop...@citrix.com; Wu, Feng <feng...@intel.com>; xen- > de...@lists.xenproject.org > Cc: k...@xen.org > Subject: Re: [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV > guest code > > >>>> Andrew Cooper <andrew.coop...@citrix.com> 03/09/16 1:33 PM >>> > >On 09/03/16 12:27, Wu, Feng wrote: > >> Oh, thanks for the clarification! Do you know how "An NMI or #MC may > occur > >> between clearing CR4.SMEP and CR4.SMAP in compat_restore_all_guest and > >> it actually returning to guest context, in which case the guest would run > >> with > >> the two features enabled. " can happen? Especially how the guest can run > >> with the two features enabled? > > > >NMIs and MCEs can occur at any point, even if interrupts are disabled. > > > >The bad situation is this sequence: > > > >* Xen is returning to the guest and disables CR4.SMEP/SMAP > >* NMI occurs while still in Xen > >* NMI exit path sees it is returning to Xen and re-enabled CR4.SMEP/SMAP > > Well, almost: Re-enabling happens on the NMI entry path. The NMI exit > path would, seeing it's returning to Xen context, simply not disable them > again.
Oh, this is the point I ignored. Thanks for the clarification. Thanks, Feng > > Jan > > >* Xen ends up returning to guest with CR4.SMEP/SMAP enabled. > > > >~Andrew > _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
