linux-kernel - Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANDhNCpi_MyGWH2jZcSRB4RU28Ga08Cqm8cyY_6wkZhNMJsNSQ@mail.gmail.com>
Date: Tue, 25 Jun 2024 15:22:00 -0700
From: John Stultz <jstultz@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Peter Hilber <peter.hilber@...nsynergy.com>, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org, 
	"Ridoux, Julien" <ridouxj@...zon.com>, virtio-dev@...ts.linux.dev, 
	"Luu, Ryan" <rluu@...zon.com>, "Christopher S. Hall" <christopher.s.hall@...el.com>, 
	Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, 
	Richard Cochran <richardcochran@...il.com>, Stephen Boyd <sboyd@...nel.org>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Marc Zyngier <maz@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Alessandro Zummo <a.zummo@...ertech.it>, Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

On Tue, Jun 25, 2024 at 2:48 PM David Woodhouse <dwmw2@...radead.org> wrote:
> On Tue, 2024-06-25 at 23:34 +0200, Thomas Gleixner wrote:
> > On Tue, Jun 25 2024 at 20:01, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@...zon.co.uk>
> > >
> > > The vmclock "device" provides a shared memory region with precision clock
> > > information. By using shared memory, it is safe across Live Migration.
> > >
> > > Like the KVM PTP clock, this can convert TSC-based cross timestamps into
> > > KVM clock values. Unlike the KVM PTP clock, it does so only when such is
> > > actually helpful.
> > >
> > > The memory region of the device is also exposed to userspace so it can be
> > > read or memory mapped by application which need reliable notification of
> > > clock disruptions.
> >
> > There is effort underway to expose PTP clocks to user space via VDSO.
>
> Ooh, interesting. Got a reference to that please?
>
> >  Can we please not expose an ad hoc interface for that?
>
> Absolutely. I'm explicitly trying to intercept the virtio-rtc
> specification here, to *avoid* having to do anything ad hoc.
>
> Note that this is a "vDSO-style" interface from hypervisor to guest via
> a shared memory region, not necessarily an actual vDSO.
>
> But yes, it *is* intended to be exposed to userspace, so that userspace
> can know the *accurate* time without a system call, and know that it
> hasn't been perturbed by live migration.

Yea, I was going to raise a concern that just defining an mmaped
structure means it has to trust the guest logic is as expected. It's
good that it's versioned! :)

I'd fret a bit about exposing this to userland. It feels very similar
to the old powerpc systemcfg implementation that similarly mapped just
kernel data out to userland and was difficult to maintain as changes
were made. Would including a code page like a proper vdso make sense
to make this more flexible of an UABI to maintain?

thanks
-john