On Mon, Oct 31, 2016 at 07:31:53PM -0400, Paolo Bonzini wrote:
>
> > In commit 108b249c453dd7132599ab6dc7e435a7036c193f, "KVM: x86: introduce
> > get_kvmclock_ns", a function to obtain the time as would be done by the
> > guest via kvmclock was introduced and used throughout the code.
> >
> > The problem is that it reads the guest TSC value via kvm_read_l1_tsc
> > which can only be called in vCPU context as it reads VMCS(TSC_OFFSET)
> > directly (on Intel CPUs). Therefore, when called in kvm_arch_vm_ioctl
> > for KVM_[GS]ET_CLOCK, it returns bogus values, breaking save/restore.
>
> The patch isn't good unfortunately, because only __get_kvmclock_ns
> can return the pvclock value.
Agreed.
> However, after Luiz's commit a545ab6a0085
> ("kvm: x86: add tsc_offset field to struct kvm_vcpu_arch", 2016-09-07)
> the TSC offset is cached outside the VMCS/VMCB
I was tempted to use it but saw it not updated in
adjust_tsc_offset_* and thought it had some different meaning.
Apparently that was just a bug which you also fix in your patch by
making adjust_tsc_offset_guest go through kvm_vcpu_write_tsc_offset.
> and all the complexity in
> vmx.c and (to a lesser extent) svm.c can be dropped. See the following
> untested patch...
>
Consider it
Reviewed-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
Tested-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
Thanks,
Roman.
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