linux-kernel - Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

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Message-ID: <h47uwzno7oqer72sjwyc4spxaduggqi4meccjvai6v42iwnqnh@uhloooww25mo>
Date: Fri, 27 Jun 2025 13:25:12 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>, 
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Brijesh Singh <brijesh.singh@....com>, 
	Michael Roth <michael.roth@....com>, Tony Luck <tony.luck@...el.com>, 
	Alexey Kardashevskiy <aik@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>, 
	Ingo Molnar <mingo@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	Sandipan Das <sandipan.das@....com>, Breno Leitao <leitao@...ian.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Alexei Starovoitov <ast@...nel.org>, 
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	Andrew Morton <akpm@...ux-foundation.org>, Luis Chamberlain <mcgrof@...nel.org>, 
	Yuntao Wang <ytcoode@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Tejun Heo <tj@...nel.org>, Changbin Du <changbin.du@...wei.com>, 
	Huang Shijie <shijie@...amperecomputing.com>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Namhyung Kim <namhyung@...nel.org>, Arnaldo Carvalho de Melo <acme@...hat.com>, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, 
	linux-mm@...ck.org, Yian Chen <yian.chen@...el.com>
Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On Thu, Jun 26, 2025 at 10:21:23AM -0700, Dave Hansen wrote:
> On 6/26/25 09:07, Borislav Petkov wrote:
> >> Makes more sense?
> > I meant this crap, ofc:
> > 
> >         switch (bug) {
> >         case X86_BUG_CPU_MELTDOWN:
> >                 if (boot_cpu_has(X86_FEATURE_PTI))
> >                         return sysfs_emit(buf, "Mitigation: PTI\n");
> > 
> > This should say "Mitigation: LASS" if LASS is enabled...
> > 
> > Which begs the question: how do LASS and PTI interact now?
> 
> Maybe my babbling about LASS mitigation Meltdown was ill considered. It
> seems that I've just muddied the waters.
> 
> All the real LASS-capable hardware also has RDCL_NO=1 which is the
> _actual_ x86 Meltdown mitigation. Those systems are not vulnerable to
> Meltdown in the first place.
> 
> They should say: "Not affected" for Meltdown, both before and after LASS.

Right. To best of my knowledge, SLAM is the only known vulnerability LASS
fixes directly so far.

So, we want an entry for SLAM?

I don't think it is very useful as we don't allow LAM if LASS is missing.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov