Re: [Xen-devel] [PATCH v2 2/3] x86/svm: Always intercept ICEBP

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 26.11.2019 14:03, Andrew Cooper wrote:
> ICEBP isn't handled well by SVM.
> 
> The VMexit state for a #DB-vectored TASK_SWITCH has %rip pointing to the
> appropriate instruction boundary (fault or trap, as appropriate), except for
> an ICEBP-induced #DB TASK_SWITCH, where %rip points at the ICEBP instruction
> rather than after it.  As ICEBP isn't distinguished in the vectoring event
> type, the state is ambiguous.
> 
> To add to the confusion, an ICEBP which occurs due to Introspection
> intercepting the instruction, or from x86_emulate() will have %rip updated as
> a consequence of partial emulation required to inject an ICEBP event in the
> first place.
> 
> We could in principle spot the non-injected case in the TASK_SWITCH handler,
> but this still results in complexity if the ICEBP instruction also has an
> Instruction Breakpoint active on it (which genuinely has fault semantics).
> 
> Unconditionally intercept ICEBP.  This does have a trap semantics for the
> intercept, and allows us to move %rip forwards appropriately before the
> TASK_SWITCH intercept is hit.  This makes the behaviour of #DB-vectored
> switches consistent however the ICEBP #DB came about, and avoids special cases
> in the TASK_SWITCH intercept.
> 
> This in turn allows for the removal of the conditional
> hvm_set_icebp_interception() logic used by the monitor subsystem, as ICEBP's
> will now always be submitted for monitoring checks.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

LGTM.

Reviewed-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
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