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Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
To:     Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Date:   Tue, 6 Mar 2018 05:45:56 -0800
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On 03/05/2018 11:04 PM, Ingo Molnar wrote:
> * H. Peter Anvin <hpa@zytor.com> wrote:
>> On NX-enabled hardware NX works with PDE, but the PDPDT in general doesn't
>> have permission bits (it's really more of a set of four CR3s than a page
>> table level.)
> The 4 PDPDT entries are also shadowed in the CPU and are only refreshed
> on CR3 loads, not spontaneously reloaded from memory during TLB walk
> like regular page table entries, right?

Yes.  The SDM even calls them non-architectural "PDPTE Registers" and
talks about them only being loaded at CR3 write time.

~5 years ago we even had a bug directly related to this feature:

> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?id=324cdc3f7e6a752fe0e95fa7b5c9664171a34ded