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Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior
- To: Ian Jackson <iwj@xxxxxxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 4 Mar 2021 16:13:15 +0100
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- Cc: <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- Delivery-date: Thu, 04 Mar 2021 15:13:39 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu, Mar 04, 2021 at 02:59:38PM +0000, Ian Jackson wrote:
> Roger Pau Monne writes ("[PATCH v2 for-4.15] x86/msr: introduce an option for
> HVM relaxed rdmsr behavior"):
> > Introduce an option to allow selecting a less strict behaviour for
> > rdmsr accesses targeting a MSR not explicitly handled by Xen. Since
> > commit 84e848fd7a162f669 accesses to MSRs not explicitly handled by
> > Xen result in the injection of a #GP to the guest. This is a behavior
> > change since previously a #GP was only injected if accessing the MSR
> > on the real hardware will also trigger a #GP.
> ...
> > I wonder whether we need to to enable this option by default for
> > guests being migrated from previous Xen versions? Maybe that's not
> > required as the option is helpful mostly for early boot I would
> > assume, afterwards an OS should already have the #GP handler setup
> > when accessing MSRs.
>
> I think it's almost as bad to have guests which can be migrated in,
> but which then cannot reboot.
Ups, yes, right.
> Historically we have taken the view that new Xen must support old
> guests, even if that means being bug-compatible. So I am strongly in
> favour of avoiding such a usability regression.
I'm not a xl/libxl expert, but couldn't we set the option in a
persistent way for migrated-in guests?
IIRC at domain creation libxl knows whether it's a restore or a fresh
domain, and hence we could set the option there?
The part I'm not sure is about how to make it persistent.
Thanks, Roger.
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