Messages in this thread | | | Date | Sun, 11 Nov 2007 11:15:51 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH 1/3] include files for kvmclock |
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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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