Re: [PATCH] Add Documentation/kvm/msr.txt
From: Zachary Amsden
Date: Thu May 27 2010 - 17:02:42 EST
On 05/27/2010 10:36 AM, Glauber Costa wrote:
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.
Little-endian non-atomic page crossing writes will write the small part
of the wallclock first, so another CPU may observe the following
wallclock sequence:
0x01ff .. 0x0100 .. 0x0200
Big-endian writes also have similar failure:
0x01ff .. 0x02ff .. 0x0200
This won't happen if there is a single instruction write of the wall
clock word.
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