Unlike 32-bit PV guests, HVM guests may switch freely between 64-bit and
other modes.  This in particular means that they may set registers used
to pass 32-bit-mode hypercall arguments to values outside of the range
32-bit code would be able to set them to.

When processing of hypercalls takes a considerable amount of time,
the hypervisor may choose to invoke a hypercall continuation.  Doing so
involves putting (perhaps updated) hypercall arguments in respective
registers.  For guests not running in 64-bit mode this further involves
a certain amount of translation of the values.

Unfortunately internal sanity checking of these translated values
assumes high halves of registers to always be clear when invoking a
hypercall.  When this is found not to be the case, it triggers a
consistency check in the hypervisor and causes a crash.

https://xenbits.xenproject.org/xsa/advisory-454.html