From: Dave Hansen <dave.hansen@intel.com>
To: Vlastimil Babka <vbabka@suse.cz>, Dave Hansen <dave@sr71.net>,
linux-kernel@vger.kernel.org
Cc: x86@kernel.org, linux-mm@kvack.org,
torvalds@linux-foundation.org, akpm@linux-foundation.org,
bp@alien8.de, ak@linux.intel.com, mhocko@suse.com,
dave.hansen@linux.intel.com,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
David Vrabel <david.vrabel@citrix.com>,
Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE
Date: Thu, 14 Jul 2016 07:24:34 -0700 [thread overview]
Message-ID: <5787A0A2.4070406@intel.com> (raw)
In-Reply-To: <71d7b63a-45dd-c72d-a277-03124b0053ae@suse.cz>
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.
next prev parent reply other threads:[~2016-07-14 14:24 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-08 0:19 [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum Dave Hansen
2016-07-08 0:19 ` [PATCH 1/4] x86, swap: move swap offset/type up in PTE to work around erratum Dave Hansen
2016-07-13 8:03 ` [tip:x86/mm] x86/mm: Move " tip-bot for Dave Hansen
2016-07-13 15:19 ` [PATCH 1/4] x86, swap: move " Michal Hocko
2016-07-08 0:19 ` [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none() Dave Hansen
2016-07-13 8:03 ` [tip:x86/mm] x86/mm: Ignore " tip-bot for Dave Hansen
2016-07-13 15:21 ` [PATCH 2/4] x86, pagetable: ignore " Michal Hocko
2016-07-13 15:47 ` Dave Hansen
2016-07-14 6:13 ` Michal Hocko
2016-07-08 0:19 ` [PATCH 3/4] x86: disallow running with 32-bit PTEs to work around erratum Dave Hansen
2016-07-13 8:04 ` [tip:x86/mm] x86/mm: Disallow " tip-bot for Dave Hansen
2016-07-08 0:19 ` [PATCH 4/4] x86: use pte_none() to test for empty PTE Dave Hansen
2016-07-13 8:04 ` [tip:x86/mm] x86/mm: Use " tip-bot for Dave Hansen
2016-07-13 15:18 ` [PATCH 4/4] x86: use " Michal Hocko
2016-07-13 15:23 ` Julia Lawall
2016-07-13 15:49 ` Julia Lawall
2016-07-13 16:28 ` Dave Hansen
2016-07-14 13:47 ` Vlastimil Babka
2016-07-14 14:24 ` Dave Hansen [this message]
2016-07-14 14:50 ` David Vrabel
2016-07-13 9:54 ` [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum Vlastimil Babka
-- strict thread matches above, loose matches on Subject: below --
2016-07-01 17:46 Dave Hansen
2016-07-01 17:47 ` [PATCH 4/4] x86: use pte_none() to test for empty PTE Dave Hansen
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