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Date: Thu, 17 Jun 2010 11:46:39 +0300
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [Qemu-devel] Re: [PATCH] hpet: Clean up initial hpet counter
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To: Jan Kiszka <jan.kiszka@web.de>
Cc: qemu-devel <qemu-devel@nongnu.org>

On Thu, Jun 17, 2010 at 10:42:34AM +0200, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Thu, Jun 17, 2010 at 10:30:15AM +0200, Jan Kiszka wrote:
> >>> Sorry, I lost you here. What "works for IO-based fw-cfg, but not for
> >>> MMIO-based".
> >> Undefined IO ports return -1, undefined (/wrt read access) MMIO 0. So
> >> you need to select a key that is different from both.
> >>
> > But can we rely on it? Is this defined somewhere or if it happens to be
> > the case in current qemu for x86 arch.
> 
> For x86 with its port-based access, we are on the safe side as (pre-pnp)
> device probing used to work this way. Can't tell for the other archs
> that support fw-cfg.
> 
> > 
> >>> Can you write pseudo logic of how you think it
> >>> all should work?
> >> The firmware should do this:
> >>
> >> write(CTL_BASE, FW_CFG_ID);
> >> if (read(CTL_BASE) != FW_CFG_ID)
> >> 	deal_with_old_qemu();
> >> else
> >> 	check_for_supported_keys();
> >>
> > Ah, I thought about read() returning 0/1, not key itself, so any key that
> > always existed would do.
> 
> Yes, read-back would mean returning FWCfgState::cur_entry. And that will
> be -1 when selected an invalid one.
> 
Heh, actually I have better idea. Why not advance FW_CFG_ID to version 2.

--
			Gleb.