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Re: [Qemu-devel] [PATCH v1] kvm/x86: Hyper-V tsc page setup

From: Andrey Smetanin
Subject: Re: [Qemu-devel] [PATCH v1] kvm/x86: Hyper-V tsc page setup
Date: Fri, 22 Jan 2016 14:59:37 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0.1

On 01/22/2016 02:53 PM, Paolo Bonzini wrote:

On 22/01/2016 12:31, Andrey Smetanin wrote:

Sometimes the TSC is detected to be unstable and Linux switches to
another clocksource.  At least in that case you can get a write to the
TSC page while the guest is running.
Sorry, now I got it, you mean host TSC is unstable and we should mark
guest tsc page invalid. Now I understand please ignore my prev. message.

No problem.  Anyhow yes, this is what I meant: a host write to the TSC
page, not a guest write to the TSC page MSR.

Usually it happens only at migration time to update the sequence---which
I believe your patch wasn't doing either.
QEMU saves address of page inside ->msr_hv_tsc, so at restore
QEMU sets corresponding MSR and KVM setup's tsc page again.
So migration should able to work.
 But if we tie TSC page
updates to kvm_gen_update_masterclock, we get that for free when the
migration destination calls the KVM_SET_CLOCK ioctl.


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