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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.05.18 at 14:39, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/05/18 13:14, Roger Pau Monné wrote:
>> On Tue, May 22, 2018 at 12:20:42PM +0100, Andrew Cooper wrote:
>>> 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,
>> Hm, there's no documentation at all for the 'ler' option.
> 
> :(  ISTR Jan introducing it for debugging purposes, but I've never used
> it myself.

Indeed I've been using this a couple of times to have an idea how execution
lead to the point where an exception was raised. The option pre-dates the
existence of xen-command-line.markdown by several releases.

Jan


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