Re: [Xen-devel] [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV guest code

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.03.16 at 09:09, <feng.wu@xxxxxxxxx> wrote:
>> >> +/* This mustn't modify registers other than %rax. */
>> >> +ENTRY(cr4_smep_smap_restore)
>> >> +        mov   %cr4, %rax
>> >> +        test  $X86_CR4_SMEP|X86_CR4_SMAP,%eax
>> >> +        jnz   0f
> 
> If we clear every place where we are back to 32bit pv guest,
> X86_CR4_SMEP and X86_CR4_SMAP bit should be clear
> in CR4, right?  If that is the case, we cannot jump to 0f.

I think Andrew's reply to (I think) a later mail of yours already
answered this, but just in case: We unconditionally come here
on paths that _may_ be used when entering Xen out of 32-bit
PV guest context. I.e. we do not know which state the two
flags are in.

Jan


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