Re: [Xen-devel] [PATCH 5/9] x86/vmx: Fix handing of MSR_DEBUGCTL on VMExit

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Tuesday, May 22, 2018 7:21 PM
> 
> Currently, whenever the guest writes a nonzero value to MSR_DEBUGCTL,
> Xen
> updates a host MSR load list entry with the current hardware value of
> MSR_DEBUGCTL.  This is wrong.
> 
> On VMExit, hardware automatically resets MSR_DEBUGCTL to 0.  The only
> case
> where different behaviour is needed is if Xen is debugging itself, and this
> needs setting up unconditionally for the lifetime of the VM.
> 
> The `ler` command line boolean is the only way to configure any use of
> MSR_DEBUGCTL for Xen, so tie the host load list entry to this setting in
> construct_vmcs().  Any runtime update of Xen's MSR_DEBUGCTL setting
> requires
> more complicated synchronisation across all the running VMs.
> 
> In the exceedingly common case, this avoids the unnecessary overhead of
> having
> a host load entry performing the same zeroing operation that hardware
> has
> already performed as part of the VMExit.

I didn't get "unnecessary overhead" part. if "ler' is disabled, as you
said earlier it's a bug to save/restore thus overhead doesn't matter.
If "ler" is enabled, then save/restore is anyway required then where
is saved overhead coming from?

Thanks
Kevin
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