Messages in this thread | | | Date | Thu, 27 May 2010 17:36:38 -0300 | From | Glauber Costa <> | Subject | Re: [PATCH] Add Documentation/kvm/msr.txt |
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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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