Messages in this thread | | | Date | Wed, 07 Oct 2009 23:37:47 +0200 | From | Avi Kivity <> | Subject | Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation |
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On 10/07/2009 11:19 PM, Jeremy Fitzhardinge wrote: > >> When do you copy? >> >> I'd rather have a single copy for guest and host. >> > When Xen updates the parameters normally. The interface never really > needed to share the memory between hypervisor and guest, and I think > avoiding it is a bit more robust. > > But for KVM, you already use the MSR to place the pvclock_vcpu_time_info > structure, so you could just place it in the page and use the same > memory for kernel and usermode. >
Yes.
>> If the hypervisor does a pvclock->version = somethingelse->version++ >> then the guest may get confused. But I understand you have a >> guest-private ->version? >> > The guest should never get confused by the version being changed by the > hypervisor. It's already part of the ABI. Or did you mean something else? >
If the guest does a RMW on the version, but the host does not (copying it from somewhere else), then the guest RMW can be lost.
Looking at the code, that's what kvm does:
vcpu->hv_clock.version += 2;
shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock, sizeof(vcpu->hv_clock));
so a guest-side ++version can be lost.
> I'm not sure what you mean by "guest-private version"; the versions are > always guest-private: te version is part of the pvclock structure, > which is per-vcpu, which is private to each guest. The guest nevern > maintains a separate long-term copy of the structure, only a transient > snapshot while computing time from the tsc (that's the current pvclock.c > code). >
Same for kvm. I'm not worried about cross-guest corruption, just the guest and host working together to confuse the guest.
>> No need to read them atomically. >> >> cpu1 = vgetcpu() >> hver1 = pvclock[cpu1].hver >> kver1 = pvclock[cpu1].kver >> tsc = rdtsc >> /* multipication magic with pvclock[cpu1]*/ >> cpu2 = vgetcpu() >> hver2 = pvclock[cpu2].hver >> kver2 = pvclock[cpu2].kver >> valid = cpu1 == cpu2&& hver1 == hver2&& kver1 == kver2 >> > I don't think that's necessary, but I can certainly live with it if it > makes you happier. >
I think the version issue requires it.
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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