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

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Message-ID: <30eb7680b3c7ae5370dfbf7510e664181f38b80e.camel@infradead.org>
Date: Fri, 02 Aug 2024 09:21:51 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>, Michael Kelley
 <mhklinux@...look.com>,  "lirongqing@...du.com" <lirongqing@...du.com>,
 "seanjc@...gle.com" <seanjc@...gle.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 Thu, 2024-08-01 at 20:54 +0200, Thomas Gleixner wrote:
> On Thu, Aug 01 2024 at 16:14, Michael Kelley wrote:
> > From: Thomas Gleixner <tglx@...utronix.de> Sent: Thursday, August
> > 1, 2024 7:21 AM
> > FWIW, in Hyper-V guests with the Hyper-V quirk removed, tglx's new
> > sequence does *not* stop the PIT. But this sequence does:
> > 
> > outb_p(0x30, PIT_MODE);
> > outb_p(0xff, PIT_CH0);
> > outb_p(0xff, PIT_CH0);
> > 
> > outb_p(0x30, PIT_MODE);
> > outb_p(0xff, PIT_CH0);
> 
> That works on bare metal too

What about writing *just* the MODE? The datasheet definitely seems to
be saying that should work too. And I believe it works on Hyper-V;
that's what the "workaround" has been doing, because Hyper-V actually
got it right.

> > 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?

I don't know that we care; the failure mode is basically harmless so
let's just let it get fixed in KVM rather than pandering to it?


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