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Re: [PATCH v3 10/16] hw/i386/vmport: Add support for CMD_GETTIME

From: Michael S. Tsirkin
Subject: Re: [PATCH v3 10/16] hw/i386/vmport: Add support for CMD_GETTIME
Date: Sat, 14 Mar 2020 16:56:13 -0400

On Sat, Mar 14, 2020 at 10:05:20PM +0200, Liran Alon wrote:
> Michael, you can also refer to this VMware time-keeping whitepaper:
> https://www.vmware.com/pdf/vmware_timekeeping.pdf.
> According to section "Initializing and Correcting Wall-Clock Time":
> """
> VMware Tools can also optionally be used to correct long‐term drift and
> errors by periodically
> resynchronizing the virtual machine’s clock to the host’s clock, but the
> current version at this writing is limited.
> In particular, in guest operating systems other than NetWare, it does not
> correct errors in which the guest clock
> is ahead of real time, only those in which the guest clock is behind.
> """

This talks about guest time.
What this does not mention is whether hosts need to employ any mechanisms
to synchronise wall clock between hosts.

> If I understand correctly, this seems to validate my assumption that current
> implementation for CMD_GETTIME is sufficient.

So I am concerned this does not interact well with other time sources
in QEMU. For example, it's very useful to set guest time with -rtc base

Can you use qemu_get_timedate?


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