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[Xen-devel] RE: TSC scaling and softtsc reprise, and PROPOSAL

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] RE: TSC scaling and softtsc reprise, and PROPOSAL
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 3 Aug 2009 13:19:03 -0700 (PDT)
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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FYI, I have confirmed with a VMware expert that
TSC is always emulated (unless a flag is set).

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Monday, July 27, 2009 1:55 PM
> To: Dan Magenheimer; Ian Pratt; Zhang, Xiantao; Xen-Devel (E-mail)
> Cc: John Levon; Dong, Eddie
> Subject: Re: TSC scaling and softtsc reprise, and PROPOSAL
> On 27/07/2009 18:25, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> >> I believe this affects the guest OS executing RDTSC, not
> >> guest apps, and is
> >> only to delay the TSC to not 'run past' pending timer ticks
> >> (typically where
> >> they have been delayed due to the guest being preempted).
> >> 
> >>  -- Keir
> > 
> > Could be.  The text would lead me to believe otherwise
> > though.  Read the section on "Virtual TSC" in the
> > above PDF. Specifically the Virtual TSC "advances even
> > when the the virtual CPU is not running" and "In the
> > past, this feature had sometimes been recommended to
> > improve performance of APPLICATIONS that read the
> > TSC frequently..." (my emphasis)
> Yes, then it sounds like they virtualise it for apps too. 
> Also there is an
> option to virtualise the TSC at a specified frequency -- that would be
> pretty weird if it applied only to guest-OS RDTSCs but not 
> guest-app RDTSCs.
> Interesting...
>  -- Keir

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