linux-kernel - Re: [PATCH] Add Documentation/kvm/msr.txt

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Message-ID: <20100527203638.GH3445@mothafucka.localdomain>
Date:	Thu, 27 May 2010 17:36:38 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add Documentation/kvm/msr.txt

On Thu, May 27, 2010 at 10:13:12AM -1000, Zachary Amsden wrote:
> On 05/27/2010 06:02 AM, Glauber Costa wrote:
> >On Thu, May 27, 2010 at 11:15:43AM +0300, Avi Kivity wrote:
> >>On 05/26/2010 09:04 PM, Glauber Costa wrote:
> >>>This patch adds a file that documents the usage of KVM-specific
> >>>MSRs.
> >>>
> >>Looks good.  A few comments:
> >>
> >>>+
> >>>+Custom MSR list
> >>>+--------
> >>>+
> >>>+The current supported Custom MSR list is:
> >>>+
> >>>+MSR_KVM_WALL_CLOCK:  0x11
> >>>+
> >>>+	data: physical address of a memory area.
> >>Which must be in guest RAM (i.e., don't point it somewhere random
> >>and expect the hypervisor to allocate it for you).
> >>
> >>Must be aligned to 4 bytes (we don't enforce it though).
> >I don't see the reason for it.
> >
> >If this is a requirement, our own implementation
> >is failing to meet it.
> 
> It's so the atomic write actually is atomic. 
Which atomic write? This is the wallclock, we do no atomic writes for
querying it. Not to confuse with system time (the other msr).

> Stating a 4 -byte
> alignment requirement prevents the wall clock from crossing a page
> boundary.

Yes, but why require it?

reading the wallclock is not a hot path for anybody, is usually done
just once, and crossing a page boundary here does not pose any correctness
issue.
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