Re: [PATCH 1/3] include files for kvmclock

[Avi Kivity <avi@qumranet.com>]

Gerd Hoffmann wrote:
> > +/*
> > + * Guest has page alignment and padding requirements. At the host, it will
> > + * only lead to wasted space at the vcpu struct. For this reason, the struct
> > + * is not anonymous
> > + */
> > +union kvm_hv_clock {
> > +	struct kvm_hv_clock_s {
> > +		u64 tsc_mult;
> > +		u64 now_ns;
> > +		/* That's the wall clock, not the water closet */
> > +		u64 wc_sec;
> > +		u64 last_tsc;
> > +		/* At first, we could use the tsc value as a marker, but Jeremy
> > +		 * well noted that it will cause us locking problems in 32-bit
> > +		 * sys, so we have a special version field */
> > +		u32 version;
> > +	} fields;
> > +	char page_align[PAGE_SIZE];
> > +};
> >     
>
> What is the point in using a whole page per vcpu?  You probably don't
> want struct kvm_hv_clock_s cross a page border.  Is that the only reason
> or are there other constrains too?
>   

We don't even have the cross-page-boundary constraint.

The real issue is that on i386 physical addresses are 64-bit while 
hypercall arguments are 32-bit.  A page frame number is 32-bit and so 
poses no issues.

I see two workarounds for this:
- make the first hypercall argument a u64 rather than a long (using two 
registers on i386)
- use an msr for registering the clock address

The first is more general, while the second has the nice property of 
taking care of live migration automatically.

> As the kvm clock looks quite simliar to what xen does, how about making
> the structs binary-compatible or simply reuse the xen version (struct
> vcpu_time_info in xen/interface/xen.h)?
>   

If there are no technical issues, I have no objections.

--
error compiling committee.c: too many arguments to function

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