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

[Zachary Amsden]

On 05/27/2010 02:10 PM, Glauber Costa wrote:
> On Thu, May 27, 2010 at 11:02:35AM -1000, Zachary Amsden wrote:
>    
> > 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.
> >      
> We already specify that users can only trust the value of the wallclock
> after we have an even version field.
> When we start the update, and during the time of all writes to it,
> it is odd, and thus, invalid.
> The ABI guarantees to the guest that we'll only bump version
> after we're done updating.
>    

I guess there is not a point.
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