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Peter Anvin" To: Dave Hansen , Nadav Amit , Andy Lutomirski CC: Pasha Tatashin , Linux Kernel Mailing List , linux-mm , Andrew Morton , the arch/x86 maintainers , Borislav Petkov , Christian Brauner , bristot@redhat.com, Ben Segall , Dave Hansen , dianders@chromium.org, dietmar.eggemann@arm.com, eric.devolder@oracle.com, hca@linux.ibm.com, "hch@infradead.org" , Jacob Pan , Jason Gunthorpe , jpoimboe@kernel.org, Joerg Roedel , juri.lelli@redhat.com, Kent Overstreet , kinseyho@google.com, "Kirill A. Shutemov" , lstoakes@gmail.com, mgorman@suse.de, mic@digikod.net, michael.christie@oracle.com, Ingo Molnar , mjguzik@gmail.com, "Michael S. Tsirkin" , Nicholas Piggin , "Peter Zijlstra (Intel)" , Petr Mladek , Rick P Edgecombe , Steven Rostedt , Suren Baghdasaryan , Thomas Gleixner , Uladzislau Rezki , vincent.guittot@linaro.org, vschneid@redhat.com Subject: Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks User-Agent: K-9 Mail for Android In-Reply-To: References: <20240311164638.2015063-1-pasha.tatashin@soleen.com> <20240311164638.2015063-12-pasha.tatashin@soleen.com> <3e180c07-53db-4acb-a75c-1a33447d81af@app.fastmail.com> <08EFDEDB-7BBB-4D9C-B7E5-D7370EC609BE@gmail.com> Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 090BB1C0009 X-Stat-Signature: b1h9446z19mgfxu5j94rjze8x16r8sx1 X-HE-Tag: 1710206818-893808 X-HE-Meta: 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 6b4OV3+S 1pf1s5bTdyPgqqEGVTsypUe1K3g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On March 11, 2024 5:53:33 PM PDT, Dave Hansen w= rote: >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=2E >Define "set speculatively"=2E :) > >> 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=E2=80=99s dirty flag being set (due = to the >> write on the first logical processor) and the entry=E2=80=99s R/W flag = being >> clear (due to the update to the entry on the second logical >> processor)=2E > >In other words, you'll see both a fault *AND* the dirty bit=2E The write >never retired and the dirty bit is set=2E > >Does that count as being set speculatively? > >That's just the behavior that the SDM explicitly admits to=2E Indeed; both the A and D bits are by design permissive; that is, the hardw= are can set them at any time=2E The only guarantees are: 1=2E The hardware will not set the A bit on a not present late, nor the D = bit on a read only page=2E 2=2E *Provided that the user has invalidated the page entry in the TLB*, h= ardware guarantees the respective bits will be set before a dependent memor= y access is made visible=2E Thus the bits are guaranteed to reflect a stric= t superset of operations performed architecturally=2E