Re: hw/clock: What clock rate for virt machines?

[Peter Maydell]

On Wed, 2 Sep 2020 at 18:03, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> On 9/2/20 6:49 PM, Peter Maydell wrote:
> > On Wed, 2 Sep 2020 at 17:35, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
> >> Peter said "'clock' is basically meaningless for virt machines",
> >>
> >> I understand and agree. But how to make that explicit/obvious in
> >> the code, when a device expects a clock frequency/period?
> >
> > When a particular *device* needs a clock, then presumably
> > it has a defined purpose for it, and we can pick a
> > frequency for it then.
> >
> >> See for example hw/riscv/virt.c, it uses the following (confusing
> >> to me) in virt_machine_init():
> >>
> >>    serial_mm_init(system_memory, memmap[VIRT_UART0].base,
> >>        0, qdev_get_gpio_in(DEVICE(mmio_plic), UART0_IRQ), 399193,
> >>        serial_hd(0), DEVICE_LITTLE_ENDIAN);
> >
> > In this case, the board has a model of a 16550A UART on it,
> > which uses its input clock to determine what the actual baud
> > rate is for particular guest settings of the divisor registers.
> > So we need to look at:
> >  * what does guest software expect the frequency to be?
>
> QEMU is supposed to model machine with no knowledge of the guest,
> but the virt case is a particular one indeed... as it has to know
> it is virtualized.
>
> >  * what is a "good" frequency which gives the guest the best
> >    possible choices of baud rate?
>
> I'll think about it...

My guess is that guest code assumes "same frequency an
x86 PC uses", but a risc-v person might know better...

(For QEMU I think it only makes a visible difference when
you pass a host serial port through to the guest as
otherwise we ignore whatever baud rate the guest sets.)

thanks
-- PMM