Re: [PATCH for-4.14 8/8] x86/hvm: enable emulated PIT for PVH dom0

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: xen-devel@lists.xenproject.org, Wei Liu <wl@xen.org>,
	Jan Beulich <jbeulich@suse.com>,
	paul@xen.org
Subject: Re: [PATCH for-4.14 8/8] x86/hvm: enable emulated PIT for PVH dom0
Date: Mon, 15 Jun 2020 17:47:59 +0200	[thread overview]
Message-ID: <20200615154759.GH735@Air-de-Roger> (raw)
In-Reply-To: <b68cede9-360e-cc4f-d88b-737ee83e654e@citrix.com>

On Mon, Jun 15, 2020 at 04:33:33PM +0100, Andrew Cooper wrote:
> On 12/06/2020 16:56, Roger Pau Monne wrote:
> > Some video BIOS require a PIT in order to work properly, hence classic
> > PV dom0 gets partial access to the physical PIT as long as it's not in
> > use by Xen.
> 
> Is this actually true today?

TBH I have no idea and asked the same thing myself.

> I can believe that it may have been necessary on old hardware, but the
> structure of systems has changed massively over the past 20 years, and
> the PIT is very legacy these days.

I also wondered whether video BIOSes really changed in the last 20
years, I really have no idea. FWIW, Wikipedia still lists PIT as being
used by video BIOSes on x86 systems [0] but there are no references to
specific models or video BIOSes.

Alternatively we could add this as a Xen command line option, ie:
dom0=pit or some such. It's however not very nice to not get output
because the video BIOS doesn't function properly due to lack of PIT.

Thanks, Roger.

[0] https://en.wikipedia.org/wiki/Intel_8253#IBM_PC_programming_tips_and_hints