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

Avi Kivity
2007-11-11T09:17:09Z

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