linux-kernel - Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown

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Message-ID: <Zrqh7GlPMRVOVtvY@google.com>
Date: Mon, 12 Aug 2024 16:59:40 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Michael Kelley <mhklinux@...look.com>, 
	"lirongqing@...du.com" <lirongqing@...du.com>, "kys@...rosoft.com" <kys@...rosoft.com>, 
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>, 
	"decui@...rosoft.com" <decui@...rosoft.com>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown

On Fri, Aug 02, 2024, David Woodhouse wrote:
> On Fri, 2024-08-02 at 07:55 -0700, Sean Christopherson wrote:
> > On Fri, Aug 02, 2024, David Woodhouse wrote:
> > > On Thu, 2024-08-01 at 20:54 +0200, Thomas Gleixner wrote:
> > > > On Thu, Aug 01 2024 at 16:14, Michael Kelley wrote:
> > > > > I don't have a convenient way to test my sequence on KVM.
> > > > 
> > > > But still fails in KVM
> > > 
> > > By KVM you mean the in-kernel one that we want to kill because everyone
> > > should be using userspace IRQ chips these days?
> > 
> > What exactly do you want to kill?  In-kernel local APIC obviously needs to stay
> > for APICv/AVIC.
> 
> The legacy PIT, PIC and I/O APIC.
> 
> > And IMO, encouraging userspace I/O APIC emulation is a net negative for KVM and
> > the community as a whole, as the number of VMMs in use these days results in a
> > decent amount of duplicated work in userspace VMMs, especially when accounting
> > for hardware and software quirks.
> 
> I don't particularly care, but I thought the general trend was towards
> split irqchip mode, with the local APIC in-kernel but i8259 PIC and I/O
> APIC (and the i8254 PIT, which was the topic of this discussion) being
> done in userspace.

Yeah, that's where most everyone is headed, if not already there.  Letting the
I/O APIC live in userspace is probably the right direction long term, I just don't
love that every VMM seems to have it's own slightly different version.  But I think
the answer to that is to build a library for (legacy?) device emulation so that
VMMs can link to an implementation instead of copy+pasting from somwhere else and
inevitably ending up with code that's frozen in time.