Re: [Xen-devel] [PATCH for-next] x86/svm: Correct vm_event API for descriptor accesses

[Adrian Pop <apop@xxxxxxxxxxxxxxx>]

Hello,

Andrew Cooper <andrew.cooper3@xxxxxxxxxx> writes:

> c/s d0a699a389f1 "x86/monitor: add support for descriptor access events"
> introduced logic looking for what appeared to be exitinfo (not that this
> exists in SVM - exitinfo1 or 2 do), but actually passed the exit IDT vectoring
> information.  There is never any IDT vectoring involved in these intercepts so
> the value passed is always zero.
>
> In fact, SVM doesn't provide any information, even in exitinfo1 and 2.  Note
> the error in the public API and state that this field is always 0, and drop
> the SVM logic in hvm_monitor_descriptor_access().
>
> In the SVM vmexit handler itself, optimise the switch statement by observing
> that there is a linear transformation between the SVM exit_reason and
> VM_EVENT_DESC_* values.  (Bloat-o-meter reports 6028 => 5877 for a saving of
> 151 bytes).
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> CC: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> CC: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
> CC: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
> CC: Adrian Pop <apop@xxxxxxxxxxxxxxx>
>
> Adrian: Do you recall what information you were attempting to forward from the
> VMCB?  I can't locate anything which would plausibly be interesting.

The SVM part was most likely meant to mirror the logic from VMX.  But,
as I recall, monitoring hadn't been implemented with SVM so this
couldn't really be used on AMD.  Unfortunately I'm not sure what
information was supposed to be forwarded.

The cleanup looks good to me.

Acked-by: Adrian Pop <apop@xxxxxxxxxxxxxxx>

> This is part of a longer cleanup series I gathered in the wake of the task
> switch issues, but it is pulled out ahead due to its interaction with the
> public interface.

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