Re: [PATCH 1/5] mmu: mark spte present if the x bit is set

From	Paolo Bonzini <pbonzini@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 1/5] mmu: mark spte present if the x bit is set
Date	2016-06-28 22:50 +0200
Message-ID	<rP87f-3nq-13@gated-at.bofh.it> (permalink)
References	(1 earlier) <rOSYx-1Xl-5@gated-at.bofh.it> <rOWSt-4vW-9@gated-at.bofh.it> <rP59o-1Ba-15@gated-at.bofh.it> <rP7Ee-3d8-37@gated-at.bofh.it> <rP7Xz-3k5-23@gated-at.bofh.it>
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On 28/06/2016 22:37, Bandan Das wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> On 28/06/2016 19:33, Bandan Das wrote:
>>>>>>>  static int is_shadow_present_pte(u64 pte)
>>>>>>>  {
>>>>>>> -	return pte & PT_PRESENT_MASK && !is_mmio_spte(pte);
>>>>>>> +	return pte & (PT_PRESENT_MASK | shadow_x_mask) &&
>>>>>>> +		!is_mmio_spte(pte);
>>>>>
>>>>> This should really be pte & 7 when using EPT.  But this is okay as an
>>>>> alternative to a new shadow_present_mask.
>>> I could revive shadow_xonly_valid probably... Anyway, for now I will
>>> add a TODO comment here.
>>
>> It's okay to it like this, because the only invalid PTEs reaching this
>> point are those that is_mmio_spte filters away.  Hence you'll never get
>> -W- PTEs here, and pte & 7 is really the same as how you wrote it.  It's
>> pretty clever, and doesn't need a TODO at all. :)
> 
> Thanks, understood. So, the way it is written now covers all cases for
> pte & 7. Let's still add a comment - clever things are usually
> confusing to many!

I think another way to write it is "(pte & 0xFFFFFFFFull) &&
!is_mmio_spte(pte)", since non-present/non-MMIO SPTEs never use bits
1..31 (they can have non-zero bits 32..63 on 32-bit CPUs where we don't
update the PTEs atomically).  Guangrong, what do you prefer?

Paolo

Thread

[PATCH 1/5] mmu: mark spte present if the x bit is set Bandan Das <bsd@redhat.com> - 2016-06-28 06:40 +0200
  Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Paolo Bonzini <pbonzini@redhat.com> - 2016-06-28 10:50 +0200
    Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Bandan Das <bsd@redhat.com> - 2016-06-28 19:40 +0200
      Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Paolo Bonzini <pbonzini@redhat.com> - 2016-06-28 22:20 +0200
        Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Bandan Das <bsd@redhat.com> - 2016-06-28 22:40 +0200
          Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Paolo Bonzini <pbonzini@redhat.com> - 2016-06-28 22:50 +0200
            Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Bandan Das <bsd@redhat.com> - 2016-06-28 23:10 +0200
            Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-06-29 05:10 +0200
            Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Wanpeng Li <kernellwp@gmail.com> - 2016-07-05 05:10 +0200
              Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Paolo Bonzini <pbonzini@redhat.com> - 2016-07-05 13:00 +0200
                Re: [PATCH 1/5] mmu: mark spte present if the x bit is set Wanpeng Li <kernellwp@gmail.com> - 2016-07-05 13:40 +0200

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