Re: [Xen-devel] [PATCH v2 3/3] x86/svm: Write the correct %eip into the outgoing task

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.11.2019 13:03, Andrew Cooper wrote:
> The TASK_SWITCH vmexit has fault semantics, and doesn't provide any NRIPs
> assistance with instruction length.  As a result, any instruction-induced task
> switch has the outgoing task's %eip pointing at the instruction switch caused
> the switch, rather than after it.
> 
> This causes callers of task gates to livelock (repeatedly execute the call/jmp
> to enter the task), and any restartable task to become a nop after its first
> use (the (re)entry state points at the ret/iret used to exit the task).
> 
> 32bit Windows in particular is known to use task gates for NMI handling, and
> to use NMI IPIs.
> 
> In the task switch handler, distinguish instruction-induced from
> interrupt/exception-induced task switches, and decode the instruction under
> %rip to calculate its length.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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