On 13/06/19 19:02, Jim Mattson wrote:
> On Tue, May 7, 2019 at 8:36 AM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
>
>> Not intercepting fields tagged read-only also allows for additional
>> optimizations, e.g. marking GUEST_{CS,SS}_AR_BYTES as SHADOW_FIELD_RO
>> since those fields are rarely written by a VMMs, but read frequently.
>
> Do you have data to support this, or is this just a gut feeling? The
> last time I looked at Virtual Box (which was admittedly a long time
> ago), it liked to read and write just about every VMCS guest-state
> field it could find on every VM-exit.
I have never looked at VirtualBox, but most other hypervisors do have a
common set of fields (give or take a couple) that they like to read
and/or write on most if not every vmexit.
Also, while this may vary dynamically based on the L2 guest that is
running, this is much less true for unrestricted-guest processors.
Without data on _which_ scenarios are bad for a static set of shadowed
fields, I'm not really happy to add even more complexity.
Paolo
> The decision of which fields to shadow is really something that should
> be done dynamically, depending on the behavior of the guest hypervisor
> (which may vary depending on the L2 guest it's running!) Making the
> decision statically is bound to result in a poor outcome for some
> scenarios.
