On Mon, Dec 03, 2018 at 02:18:25PM +0000, Marc Zyngier wrote:
> On 03/12/2018 13:50, Andrew Jones wrote:
> > On Wed, Nov 28, 2018 at 02:45:16PM +0000, Steven Price wrote:
> >> +The structure pointed to by the PV_TIME_LPT hypercall is as follows:
> >> +
> >> + Field | Byte Length | Byte Offset | Description
> >> + --------------- | ----------- | ----------- | -------------------------------
> >> + Revision | 4 | 0 | Must be 0 for this revision
> >> + Attributes | 4 | 4 | Must be 0
> >> + sequence_number | 8 | 8 | Bit 0: reserved
> >> + | | | Bits 1:63 number of migrations
> >> + scale_mult | 8 | 16 | Multiplier to scale from native
> >> + | | | frequency to PV frequency
> >> + shift | 4 | 24 | Shift applied before multiplier
> >> + Reserved | 4 | 28 | Must be 0
> >> + Fn | 8 | 32 | Native frequency
> >> + Fpv | 8 | 40 | Paravirtualized frequency seen
> >> + | | | by guest
> >> + div_by_fpv_mult | 8 | 48 | Multiplier to implement fast
> >> + | | | divide by Fpv
> >
> > Here's kvmclock's struct
> >
> > struct pvclock_vcpu_time_info {
> > u32 version;
> > u32 pad0;
> > u64 tsc_timestamp;
> > u64 system_time;
> > u32 tsc_to_system_mul;
> > s8 tsc_shift;
> > u8 flags;
> > u8 pad[2];
> > }
> >
> > Revision =>
> > Attributes =>
> > sequence_number => version
> > scale_mult => tsc_to_system_mul (this is reversed, but OK)
> > shift => tsc_shift (also reversed)
> > Reserved =>
> > Fn => (pvclock doesn't have, but does have system_time)
> > Fpv =>
> > div_by_fpv_mult =>
> >
> > I haven't thought about this enough yet to be sure kvmclock's fields
> > are sufficient, but several look close - although the 'tsc' naming
> > isn't nice. Also, the pvclock struct could be extended by adding an
> > 'extended' flag to 'flags', and then appending more fields.
>
> One important thing is that this is not a KVM PV time implementation.
> This is something generic for the ARM architecture. Any ARM hypervisor
> should be able to implement this.
>
I agree with that, but xen also uses the same pvclock structure on x86.
Thanks,
drew
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