linux-kernel - Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks

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Message-ID: <dd67a2b9-35ef-4cf8-b303-9e6b8692b390@intel.com>
Date: Mon, 11 Mar 2024 17:53:33 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Nadav Amit <nadav.amit@...il.com>, Andy Lutomirski <luto@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 linux-mm <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>,
 the arch/x86 maintainers <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
 Christian Brauner <brauner@...nel.org>, bristot@...hat.com,
 Ben Segall <bsegall@...gle.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 dianders@...omium.org, dietmar.eggemann@....com, eric.devolder@...cle.com,
 hca@...ux.ibm.com, "hch@...radead.org" <hch@...radead.org>,
 "H. Peter Anvin" <hpa@...or.com>, Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Jason Gunthorpe <jgg@...pe.ca>, jpoimboe@...nel.org,
 Joerg Roedel <jroedel@...e.de>, juri.lelli@...hat.com,
 Kent Overstreet <kent.overstreet@...ux.dev>, kinseyho@...gle.com,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, lstoakes@...il.com,
 mgorman@...e.de, mic@...ikod.net, michael.christie@...cle.com,
 Ingo Molnar <mingo@...hat.com>, mjguzik@...il.com,
 "Michael S. Tsirkin" <mst@...hat.com>, Nicholas Piggin <npiggin@...il.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Petr Mladek <pmladek@...e.com>, Rick P Edgecombe
 <rick.p.edgecombe@...el.com>, Steven Rostedt <rostedt@...dmis.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Uladzislau Rezki <urezki@...il.com>,
 vincent.guittot@...aro.org, vschneid@...hat.com
Subject: Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks

On 3/11/24 16:56, Nadav Amit wrote:
> So you can look on the dirty-bit, which is not being set
> speculatively and save yourself one problem.
Define "set speculatively". :)

> If software on one logical processor writes to a page while software
> on another logical processor concurrently clears the R/W flag in the
> paging-structure entry that maps the page, execution on some
> processors may result in the entry’s dirty flag being set (due to the
> write on the first logical processor) and the entry’s R/W flag being
> clear (due to the update to the entry on the second logical
> processor).

In other words, you'll see both a fault *AND* the dirty bit.  The write
never retired and the dirty bit is set.

Does that count as being set speculatively?

That's just the behavior that the SDM explicitly admits to.