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[79.180.7.202]) by smtp.gmail.com with ESMTPSA id h25sm11623507qkg.87.2020.03.14.13.56.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 14 Mar 2020 13:56:17 -0700 (PDT) Date: Sat, 14 Mar 2020 16:56:13 -0400 From: "Michael S. Tsirkin" To: Liran Alon Subject: Re: [PATCH v3 10/16] hw/i386/vmport: Add support for CMD_GETTIME Message-ID: <20200314164850-mutt-send-email-mst@kernel.org> References: <0bc7dac9-41a6-5af2-a1d5-ef1e79f92eea@oracle.com> <20200313113049-mutt-send-email-mst@kernel.org> <3c0d9308-f56c-0766-9815-241a28d9a246@oracle.com> <20200313170914-mutt-send-email-mst@kernel.org> <67d158f3-9d6c-cba8-6b3a-87702abdc6f0@oracle.com> <20200314151236-mutt-send-email-mst@kernel.org> <20200314152119-mutt-send-email-mst@kernel.org> <5f9f6b13-d382-b9a6-52cd-80014c2f01b8@oracle.com> MIME-Version: 1.0 In-Reply-To: <5f9f6b13-d382-b9a6-52cd-80014c2f01b8@oracle.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: ehabkost@redhat.com, qemu-devel@nongnu.org, Nikita Leshenko , pbonzini@redhat.com, rth@twiddle.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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=E2=80=90term dri= ft and > errors by periodically > resynchronizing the virtual machine=E2=80=99s clock to the host=E2=80=99s= 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. >=20 > """ 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 curr= ent > 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 flag. Can you use qemu_get_timedate? --=20 MST