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
signature.asc
Description: PGP signature
