Re: [PATCH] hw/ppc: Fix clock update drift

[Cédric Le Goater]

On 6/29/23 09:04, Frederic Barrat wrote:
> On 29/06/2023 04:07, Nicholas Piggin wrote:
> > The clock update logic reads the clock twice to compute the new clock
> > value, with a value derived from the later time subtracted from a value
> > derived from the earlier time. The delta causes time to be lost.
> >
> > This can ultimately result in time becoming unsynchronized between CPUs
> > and that can cause OS lockups, timeouts, watchdogs, etc. This can be
> > seen running a KVM guest (that causes lots of TB updates) on a powernv
> > SMP machine.
> >
> > Fix this by reading the clock once.
> >
> > Cc: qemu-stable@nongnu.org
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
>
>
> Nice! It must have been a nightmare to debug.
> Do you know where that ATB spr comes from? I can't relate it to the ISA.

You should look at the e500 CPU (BookE) specs.

C.