linux-kernel - Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority management)

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <66915ef3.050a0220.72f83.316b@mx.google.com>
Date: Fri, 12 Jul 2024 12:50:57 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Vineeth Remanan Pillai <vineeth@...byteword.org>,
	Ben Segall <bsegall@...gle.com>, Borislav Petkov <bp@...en8.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	"H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Wanpeng Li <wanpengli@...cent.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Masami Hiramatsu <mhiramat@...nel.org>, himadrics@...ia.fr,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	graf@...zon.com, drjunior.org@...il.com
Subject: Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority
 management)

On Fri, Jul 12, 2024 at 09:44:16AM -0700, Sean Christopherson wrote:
> On Fri, Jul 12, 2024, Steven Rostedt wrote:
> > On Fri, 12 Jul 2024 10:09:03 -0400
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > 
> > > > 
> > > > Steven Rostedt told me, what we instead need is a tracepoint callback in a
> > > > driver, that does the boosting.  
> > > 
> > > I utterly dislike changing the system behavior through tracepoints. They were
> > > designed to observe the system, not modify its behavior. If people start abusing
> > > them, then subsystem maintainers will stop adding them. Please don't do that.
> > > Add a notifier or think about integrating what you are planning to add into the
> > > driver instead.
> > 
> > I tend to agree that a notifier would be much better than using
> > tracepoints, but then I also think eBPF has already let that cat out of
> > the bag. :-p
> > 
> > All we need is a notifier that gets called at every VMEXIT.
> 
> Why?  The only argument I've seen for needing to hook VM-Exit is so that the
> host can speculatively boost the priority of the vCPU when deliverying an IRQ,
> but (a) I'm unconvinced that is necessary, i.e. that the vCPU needs to be boosted
> _before_ the guest IRQ handler is invoked and (b) it has almost no benefit on
> modern hardware that supports posted interrupts and IPI virtualization, i.e. for
> which there will be no VM-Exit.

I am a bit confused by your statement Sean, because if a higher prio HOST
thread wakes up on the vCPU thread's phyiscal CPU, then a VM-Exit should
happen. That has nothing to do with IRQ delivery.  What am I missing?

thanks,

 - Joel