Re: [Xen-devel] [edk2-devel] [PATCH v4 29/35] OvmfPkg/OvmfXen: Override PcdFSBClock to Xen vLAPIC timer frequency

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Aug 08, 2019 at 03:26:41PM +0100, Anthony PERARD wrote:
> On Thu, Aug 08, 2019 at 03:44:23PM +0200, Roger Pau Monné wrote:
> > On Thu, Aug 08, 2019 at 02:28:32PM +0100, Anthony PERARD wrote:
> > > On Wed, Aug 07, 2019 at 05:54:51PM +0200, Roger Pau Monné wrote:
> > > > On Mon, Jul 29, 2019 at 04:39:38PM +0100, Anthony PERARD wrote:
> > > > > PcdFSBClock is used by SecPeiDxeTimerLibCpu, the TimerLib
> > > > > implementation. It will also be used by XenTimerDxe. Override
> > > > > PcdFSBClock to match Xen vLAPIC timer frequency.
> > > > 
> > > > I'm kind of surprised that his is not automatically detected?
> > > > 
> > > > Is it a bug in Xen or just always hardcoded on OVMF?
> > > 
> > > It's hardcoded. Why would you need auto detection when you always run on
> > > the same machine ;-).
> > 
> > I don't think that's part of the HVM/PVH ABI in any way, and if you
> > hardcode it in OVMF it would prevent Xen from changing the frequency
> > in the future if such necessity arises. We should try to avoid painting
> > ourselves into a corner when possible.
> > 
> > Doesn't OVMF have a way to get this from the hardware itself?
> 
> So EDKII doesn't have that capability, FSBClock is a build time value
> and can't be changed at run time. But OVMF (on KVM or HVM) doesn't use
> that value at all, it uses the ACPI timer instead.

But after your series both PVH and HVM will use the lapic timer
instead of the ACPI timer, and thus rely on the value of FSBClock?

> We could try to find a better way to get time in OvmfXen without
> hardcoding FSBClock, but maybe in a follow-up.

Yes please. As said above lapic frequency is not part of the ABI, so
if we need to use it like that APIC_BUS_CYCLE_NS (currently in
vlapic.h) would have to be moved to a public header.

Thanks, Roger.

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