Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	x86@kernel.org, Kostya Serebryany <kcc@google.com>,
	Andrey Ryabinin <ryabinin.a.a@gmail.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	"H . J . Lu" <hjl.tools@gmail.com>,
	Andi Kleen <ak@linux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR
Date: Wed, 29 Jun 2022 03:53:42 +0300	[thread overview]
Message-ID: <20220629005342.3thjt26e6p6znyrh@black.fi.intel.com> (raw)
In-Reply-To: <6cb17661-9436-afbf-38eb-58565bba1a56@kernel.org>

On Tue, Jun 28, 2022 at 04:42:40PM -0700, Andy Lutomirski wrote:
> On 6/10/22 07:35, Kirill A. Shutemov wrote:
> 
> > +	/* Update CR3 to get LAM active */
> > +	switch_mm(current->mm, current->mm, current);
> 
> Can you at least justify this oddity?  When changing an LDT, we use a
> dedicated mechanism.  Is there a significant benefit to abusing switch_mm
> for this?

I'm not sure I follow. LAM mode is set in CR3. switch_mm() has to handle
it anyway to context switch. Why do you consider it abuse?

> 
> Also, why can't we enable LAM on a multithreaded process?  We can change an
> LDT, and the code isn't even particularly complicated.

I reworked this in v4[1] and it allows multithreaded processes. Have you
got that version?

Intel had issue with mail server, but I assumed it didn't affect my
patchset since I see it in the archive.

[1] https://lore.kernel.org/all/20220622162230.83474-1-kirill.shutemov@linux.intel.com/

-- 
 Kirill A. Shutemov