Re: [RFC 00/10] hw/mos6522: VIA timer emulation fixes and improvements

[David Gibson]

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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