linux-kernel - Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE

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Message-ID: <5787A6A2.3000807@citrix.com>
Date:	Thu, 14 Jul 2016 15:50:10 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>, Dave Hansen <dave@...1.net>,
	<linux-kernel@...r.kernel.org>
CC:	<x86@...nel.org>, <linux-mm@...ck.org>,
	<torvalds@...ux-foundation.org>, <akpm@...ux-foundation.org>,
	<bp@...en8.de>, <ak@...ux.intel.com>, <mhocko@...e.com>,
	<dave.hansen@...ux.intel.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE

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@...rix.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