From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <464C51F7.3000407@domain.hid> Date: Thu, 17 May 2007 15:00:39 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <17996.15540.824415.263562@domain.hid> <464C4DE4.1040402@domain.hid> <17996.20586.564091.503622@domain.hid> In-Reply-To: <17996.20586.564091.503622@domain.hid> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig0B19C311240EA908608D4751" Sender: jan.kiszka@domain.hid Subject: Re: [Xenomai-core] [PATCH] Use tsc for implementation of clock_gettime. List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: xenomai@xenomai.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig0B19C311240EA908608D4751 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Gilles Chanteperdrix wrote: > Jan Kiszka wrote: > > Gilles Chanteperdrix wrote: > > > Hi, > > >=20 > > > here comes, for review, a patch which reduces the overhead of > > > clock_gettime by directly reading the tsc in user-space for > > > architectures that support it. > >=20 > > Highly welcome. But I have one concern: How and when do you propagat= e > > wallclock_offset changes to user space? >=20 > Since clock_settime is not implemented, never, but if clock_settime was= > implemented, clock_settime would re-issue the __xn_sys_info syscall. This excludes automatic clock adjustment, something I'm convinced we will have to provide in the future. >=20 > >=20 > > I think we need some vsyscall-alike approach for this, some read-onl= y > > page that is mapped into every RT process, containing things like a > > regularly updated offset (seqlock fashioned) or other read-only > > information (shadow mode? cpu id?). >=20 > As I said yesterday, having a page mapped at a different address in > kernel-space and user-space is not an option on ARM, because each > mapping of this page would have a separated cache. >=20 Hmm, what about the kernel writing directly to the (current) user address? We then only need to keep track of the per-mm address and have not fixed kernel equivalent. At least on arm, other archs may handle this differently at compile time. Jan --------------enig0B19C311240EA908608D4751 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.5 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iD8DBQFGTFH3niDOoMHTA+kRApmvAJ9dEyjyRw1Wo+4eKP/Th4E8LLXfOACePdXR gBQxVLITole38l+MABipOAk= =tYSf -----END PGP SIGNATURE----- --------------enig0B19C311240EA908608D4751--