Re: [RESEND] qemu/timer: Add host ticks function for RISC-V

[LIU Zhiwei]

On 2023/9/9 22:45, Palmer Dabbelt wrote:
> On Sat, 09 Sep 2023 00:18:02 PDT (-0700), pbonzini@redhat.com wrote:
> > Il sab 9 set 2023, 03:35 Atish Patra <atishp@atishpatra.org> ha scritto:
> >
> > > On Fri, Sep 8, 2023 at 3:29 AM Paolo Bonzini <pbonzini@redhat.com> 
> > > wrote:
> > > >
> > > > Queued, thanks.
> > > >
> > >
> > > I didn't realize it was already queued. Gmail threads failed me this 
> > > time.
> > > @Paolo Bonzini : Can you please drop this one as this will break as
> > > soon as the host riscv system
> > > has the latest kernel ? I have provided more details in the original
> > > thread.
> > >
> > > https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01941.html
> >
> >
> > If you have dynamic clock adjustment, does rdcycle increase with a fixed
> > frequency or does it provide the raw number of clock cycles? If the 
> > latter,
> > I agree that it should be provided by perf; but if the frequency is 
> > fixed
> > then it would be the same as rdtsc on Intel.
>
> That really depends on exactly how the system is set up, but there are 
> systems for which the rdcycle frequency changes when clock speeds 
> change and thus will produce surprising answers for users trying to 
> use rdcycle as a RTC.  We have rdtime for that, but it has other 
> problems (it's trapped and emulated in M-mode on some systems, so it's 
> slow and noisy).
>
> So we're steering folks towards perf where we can, as at least that 
> way we've got a higher-level interface we can use to describe these 
> quirks.

OK. I will send a v2 patch using rdtime.

Thanks,
Zhiwei

> > Paolo
> >
> >
> > > > Paolo
> > > >
> > > >
> > >
> > >
> > > --
> > > Regards,
> > > Atish