From: David Gibson <david@gibson.dropbear.id.au>
To: Finn Thain <fthain@linux-m68k.org>
Cc: qemu-devel@nongnu.org, qemu-ppc@nongnu.org,
Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>,
Greg Kurz <groug@kaod.org>, Laurent Vivier <laurent@vivier.eu>
Subject: Re: [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements
Date: Wed, 25 Aug 2021 13:11:25 +1000 [thread overview]
Message-ID: <YSW03VVo3jKhnj3L@yekko> (raw)
In-Reply-To: <cover.1629799776.git.fthain@linux-m68k.org>
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On Tue, Aug 24, 2021 at 08:09:36PM +1000, Finn Thain wrote:
> This is a patch series that I started last year. The aim was to try to
> get a monotonic clocksource for Linux/m68k guests. That aim hasn't been
> achieved yet (for q800 machines) but I'm submitting the patch series as
> an RFC because,
>
> - It does improve 6522 emulation fidelity.
>
> - It allows Linux/m68k to make use of the additional timer that the
> hardware indeed offers but which QEMU omits. This has several
> benefits for Linux guests [1].
>
> - I see that Mark has been working on timer emulation issues in his
> github repo [2] and it seems likely that MacOS, NetBSD or A/UX guests
> will also require better 6522 emulation.
>
> To make collaboration easier these patches can also be fetched from
> github [3].
>
> On a real Quadra, accesses to the SY6522 chips are slow because they are
> synchronous with the 783360 Hz "phase 2" clock. In QEMU, they are slow
> only because of the division operation in the timer count calculation.
>
> This patch series improves the fidelity of the emulated chip, but the
> price is more division ops. I haven't tried to measure this yet.
>
> The emulated 6522 still deviates from the behaviour of the real thing,
> however. For example, two consecutive accesses to a real 6522 timer
> counter can never yield the same value. This is not true of the 6522 in
> QEMU 6 wherein two consecutive accesses to a timer count register have
> been observed to yield the same value.
>
> Linux is not particularly robust in the face of a 6522 that deviates
> from the usual behaviour. The problem presently affecting a Linux guest
> is that its 'via' clocksource is prone to monotonicity failure. That is,
> the clocksource counter can jump backwards. This can be observed by
> patching Linux like so:
I'm afraid I know very little about Mac hardware, and almost nothing
about m68k, so I can't really review these. I have a batch of updates
for MAINTAINERS underway, which will take me off the list for this
file.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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next prev parent reply other threads:[~2021-08-25 4:20 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-24 10:09 [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements Finn Thain
2021-08-24 10:09 ` [RFC 09/10] hw/mos6522: Avoid using discrepant QEMU clock values Finn Thain
2021-08-24 10:28 ` Philippe Mathieu-Daudé
2021-08-29 1:23 ` Finn Thain
2021-08-25 8:44 ` Mark Cave-Ayland
2021-08-29 1:55 ` Finn Thain
2021-08-24 10:09 ` [RFC 06/10] hw/mos6522: Implement oneshot mode Finn Thain
2021-08-25 8:18 ` Mark Cave-Ayland
2021-08-29 1:20 ` Finn Thain
2021-08-24 10:09 ` [RFC 01/10] hw/mos6522: Remove get_load_time() methods and functions Finn Thain
2021-08-24 10:29 ` Philippe Mathieu-Daudé
2021-08-25 6:55 ` Mark Cave-Ayland
2021-08-28 1:00 ` Finn Thain
2021-08-24 10:09 ` [RFC 08/10] hw/mos6522: Call mos6522_update_irq() when appropriate Finn Thain
2021-08-24 10:22 ` Philippe Mathieu-Daudé
2021-08-25 8:26 ` Mark Cave-Ayland
2021-08-24 10:09 ` [RFC 07/10] hw/mos6522: Fix initial timer counter reload Finn Thain
2021-08-25 8:23 ` Mark Cave-Ayland
2021-08-28 0:46 ` Finn Thain
2021-08-24 10:09 ` [RFC 10/10] hw/mos6522: Synchronize timer interrupt and timer counter Finn Thain
2021-08-25 8:52 ` Mark Cave-Ayland
2021-08-26 6:43 ` Finn Thain
2021-08-24 10:09 ` [RFC 04/10] hw/mos6522: Rename timer callback functions Finn Thain
2021-08-24 10:28 ` Philippe Mathieu-Daudé
2021-08-25 7:11 ` Mark Cave-Ayland
2021-08-26 7:42 ` Philippe Mathieu-Daudé
2021-08-24 10:09 ` [RFC 02/10] hw/mos6522: Remove get_counter_value() methods and functions Finn Thain
2021-08-24 10:29 ` Philippe Mathieu-Daudé
2021-08-24 10:09 ` [RFC 05/10] hw/mos6522: Don't clear T1 interrupt flag on latch write Finn Thain
2021-08-25 7:20 ` Mark Cave-Ayland
2021-08-26 5:21 ` Finn Thain
2021-09-01 14:32 ` Laurent Vivier
2021-09-01 22:26 ` Finn Thain
2021-08-24 10:09 ` [RFC 03/10] hw/mos6522: Remove redundant mos6522_timer1_update() calls Finn Thain
2021-08-25 7:09 ` Mark Cave-Ayland
2021-08-24 10:34 ` [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements Philippe Mathieu-Daudé
2021-08-28 1:22 ` Finn Thain
2021-08-31 21:14 ` Mark Cave-Ayland
2021-08-31 22:44 ` Finn Thain
2021-09-01 7:57 ` Mark Cave-Ayland
2021-09-01 8:06 ` Mark Cave-Ayland
2021-09-10 17:29 ` Mark Cave-Ayland
2021-09-11 0:08 ` Finn Thain
2021-09-01 2:20 ` Finn Thain
2021-08-25 3:11 ` David Gibson [this message]
2021-08-25 9:10 ` Mark Cave-Ayland
2021-08-28 4:11 ` Finn Thain
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