Re: [Xen-devel] [PATCH v3 4/9] Clear AC bit in RFLAGS to protect Xen itself by SMAP

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.04.14 at 05:06, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Monday, April 28, 2014 5:58 PM
>> To: Wu, Feng
>> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
>> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx 
>> Subject: RE: [PATCH v3 4/9] Clear AC bit in RFLAGS to protect Xen itself by 
> SMAP
>> 
>> >>> On 28.04.14 at 11:43, <feng.wu@xxxxxxxxx> wrote:
>> 
>> >
>> >> -----Original Message-----
>> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Monday, April 28, 2014 5:26 PM
>> >> To: Wu, Feng
>> >> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
>> >> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx 
>> >> Subject: Re: [PATCH v3 4/9] Clear AC bit in RFLAGS to protect Xen itself 
>> >> by
>> > SMAP
>> >>
>> >> >>> On 28.04.14 at 05:15, <feng.wu@xxxxxxxxx> wrote:
>> >> > @@ -466,6 +468,7 @@ ENTRY(dom_crash_sync_extable)
>> >> >          jmp   asm_domain_crash_synchronous /* Does not return */
>> >> >
>> >> >  ENTRY(common_interrupt)
>> >> > +        ASM_CLAC
>> >> >          SAVE_ALL
>> >> >          movq %rsp,%rdi
>> >> >          callq do_IRQ
>> >> > @@ -485,6 +488,7 @@ ENTRY(page_fault)
>> >> >          movl  $TRAP_page_fault,4(%rsp)
>> >> >  /* No special register assumptions. */
>> >> >  GLOBAL(handle_exception)
>> >> > +        ASM_CLAC
>> >> >          SAVE_ALL
>> >>
>> >> Did you check whether the addition wouldn't better go right into
>> >> SAVE_ALL?
>> >
>> > Most of them can be moved into SAVE_ALL obviously, however, there are two
>> > exceptions:
>> >
>> > 1. SAVE_ALL is not executed in the beginning of some exception handlers,
>> > such as, double_fault, nmi, etc.
>> 
>> But that's orthogonal - I didn't say that would automatically cover
>> _all_ cases.
> 
> So, do you think we can move CLAC into SAVE_ALL in these cases? Thanks!

That was actually meant for you to figure out. But since you ask back,
yes, I don't see any case where it would be outright wrong, but there
are cases where it unnecessary, i.e. parameterizing SAVE_ALL might
nevertheless be desirable.

>> > 2. We don't need CLAC in .fixup section where SAVE_ALL is used.
>> 
>> Do we not?
> 
> I think the AC bit is clear when running SAVE_ALL in the .fixup section.

Correct, but it would once again be harmless to do it here too.

Jan


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