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Subject: Re: [PATCH 2/3] x86/mm: introduce __PAGE_KERNEL_GLOBAL
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <0f8abc68-1092-1bae-d244-1adbbee455f9@linux.intel.com>
Date:   Fri, 16 Feb 2018 11:54:03 -0800
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        keescook@google.com, Hugh Dickins <hughd@google.com>,
        Juergen Gross <jgross@suse.com>, x86@kernel.org
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To:     Dave Hansen <dave.hansen@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 02/16/2018 10:25 AM, Nadav Amit wrote:
>>> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
>>> +#define __PAGE_KERNEL_GLOBAL		0
>>> +#else
>>> +#define __PAGE_KERNEL_GLOBAL		_PAGE_GLOBAL
>>> +#endif
>> ...
>>> --- a/arch/x86/mm/pageattr.c~kpti-no-global-for-kernel-mappings	=
2018-02-13 15:17:56.148210060 -0800
>>> +++ b/arch/x86/mm/pageattr.c	2018-02-13 15:17:56.153210060 =
-0800
>>> @@ -593,7 +593,8 @@ try_preserve_large_page(pte_t *kpte, uns
>>> 	 * different bit positions in the two formats.
>>> 	 */
>>> 	req_prot =3D pgprot_4k_2_large(req_prot);
>>> -	req_prot =3D pgprot_set_on_present(req_prot, _PAGE_GLOBAL | =
_PAGE_PSE);
>>> +	req_prot =3D pgprot_set_on_present(req_prot,
>>> +			__PAGE_KERNEL_GLOBAL | _PAGE_PSE);
>>> 	req_prot =3D canon_pgprot(req_prot);
>> =46rom these chunks, it seems to me as req_prot will not have the =
global bit
>> on when =E2=80=9Cnopti=E2=80=9D parameter is provided. What am I =
missing?
>=20
> That's a good point.  The current patch does not allow the use of
> _PAGE_GLOBAL via _PAGE_KERNEL_GLOBAL when =
CONFIG_PAGE_TABLE_ISOLATION=3Dy,
> but booted with nopti.  It's a simple enough fix.  Logically:
>=20
> #ifdef CONFIG_PAGE_TABLE_ISOLATION
> #define __PAGE_KERNEL_GLOBAL	static_cpu_has(X86_FEATURE_PTI) ?
> 					0 : _PAGE_GLOBAL
> #else
> #define __PAGE_KERNEL_GLOBAL	_PAGE_GLOBAL
> #endif
>=20
> But I don't really want to hide that gunk in a macro like that.  It
> might make more sense as a static inline.  I'll give that a shot and =
resent.

Since determining whether PTI is on is done in several places in the =
kernel,
maybe there should a single function to determine whether PTI is on,
something like:

static inline bool is_pti_on(void)
{
	return IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION) &&=20
		static_cpu_has(X86_FEATURE_PTI);
}