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

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Message-ID: <35624750846f564e6789c22801300a542cafa7fb.camel@infradead.org>
Date: Fri, 02 Aug 2024 16:04:22 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
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, 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.

Especially if you want to support guests with APIC IDs > 255 :)

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