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Subject: Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE
To: Dave Hansen <dave.hansen@intel.com>, Vlastimil Babka <vbabka@suse.cz>,
        Dave Hansen <dave@sr71.net>, <linux-kernel@vger.kernel.org>
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From: David Vrabel <david.vrabel@citrix.com>
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Date: Thu, 14 Jul 2016 15:50:10 +0100
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On 14/07/16 15:24, Dave Hansen wrote:
> On 07/14/2016 06:47 AM, Vlastimil Babka wrote:
>> So, this might be just because I know next to nothing about (para)virt,
>> but...
>>
>> in arch/x86/include/asm/paravirt.h, pte_val is implemented via some
>> pvops, which suggests that obtaining a pte value is different than just
>> reading it from memory. But I don't see pte_none() defined to be using
>> this on paravirt, and it shares (before patch 2/4) the "return !pte.pte"
>> implementation, AFAICS?
>>
>> So that itself is suspicious to me. And now that this patches does
>> things like this:
>>
>> -              if (pte_val(*pte)) {
>> +              if (!pte_none(*pte)) {
>>
>> So previously on paravirt these tests would read pte via the pvops, and
>> now they won't. Is that OK?
> 
> I've cc'd a few Xen guys.  I think they're the only ones that would care.
> 
> But, as far as I can tell, the Xen pte_val() will take a _PAGE_PRESENT
> PTE and muck with it.  But its answer will never differ for an all 0 PTE
> from !pte_none() because that PTE does not have _PAGE_PRESENT set.
> 
> It does seem fragile that Xen is doing it this way, but I guess it works.

Xen PV guests never plays games with non-present PTEs so, for the
series, wrt Xen:

Acked-by: David Vrabel <david.vrabel@citrix.com>

FWIW, present PTEs have a hardware-specified meaning where-as
non-present PTEs do not, so I'm not sure I'd view Xen PV guests making
this distinct as "fragile".


David