Re: [Xen-devel] [PATCH v6 05/10] Clear AC bit in RFLAGS to protect Xen itself by SMAP

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

> -----Original Message-----
> From: Andrew Cooper [mailto:amc96@xxxxxxxxxxxxxxxx] On Behalf Of
> Andrew Cooper
> Sent: Thursday, May 08, 2014 9:58 AM
> To: Wu, Feng; Jan Beulich
> Cc: ian.campbell@xxxxxxxxxx; Dong, Eddie; Nakajima, Jun; Tian, Kevin;
> xen-devel@xxxxxxxxxxxxx
> Subject: Re: [PATCH v6 05/10] Clear AC bit in RFLAGS to protect Xen itself by
> SMAP
> 
> On 08/05/2014 02:41, Wu, Feng wrote:
> >
> >> -----Original Message-----
> >> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> >> Sent: Wednesday, May 07, 2014 7:54 PM
> >> To: Jan Beulich
> >> Cc: Wu, Feng; ian.campbell@xxxxxxxxxx; Dong, Eddie; Nakajima, Jun; Tian,
> >> Kevin; xen-devel@xxxxxxxxxxxxx
> >> Subject: Re: [PATCH v6 05/10] Clear AC bit in RFLAGS to protect Xen itself 
> >> by
> >> SMAP
> >>
> >> On 07/05/14 12:40, Jan Beulich wrote:
> >>>>>> On 07.05.14 at 11:44, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>>> On 07/05/14 09:19, Feng Wu wrote:
> >>>>> @@ -673,6 +675,7 @@ ENTRY(nmi_crash)
> >>>>>          ud2
> >>>>>
> >>>>>  ENTRY(machine_check)
> >>>>> +        ASM_CLAC
> >>>> This is not needed.  the start of handle_ist_exception has a SAVE_ALL,
> >>>> which also covers the nmi entry point.
> >>>>
> >>>> On the subject of IST exceptions, perhaps the double fault explicitly
> >>>> wants a STAC to reduce the likelihood of taking a further fault while
> >>>> trying to dump state. ?
> >>> I agree. And perhaps along with do_double_fault(), fatal_trap()
> >>> should then also get a stac() added?
> >>>
> >>> Jan
> >>>
> >> With doubt_fault: being sole caller of do_double_fault(), editing the
> >> entry point in entry.S to "ASM_STAC; SAVE_ALL 0" is sufficient to avoid
> >> stac() in do_doube_fault() itself.
> > I think it's better to add "ASM_STAC" just before " call  do_double_fault".
> > Do you think this is okay, Andrew? Thanks!
> 
> I am not fussed where exactly the STAC goes in the entry point, but
> don't leave a CLAC in the SAVE_ALL.

Sure, thanks for the suggestion! I will pass 0 to SAVE_ALL. 
My point is that ASM_STAC should be deferred as much as possible.

> 
> ~Andrew

Thanks,
Feng

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