On Wed, Apr 08, 2020 at 10:23:58AM +0200, Paolo Bonzini wrote:
Page-not-present async page faults are almost a perfect match for the hardware use of #VE (and it might even be possible to let the processor deliver the exceptions).
My "async" page fault knowledge is limited, but if the desired behavior is to reflect a fault into the guest for select EPT Violations, then yes, enabling EPT Violation #VEs in hardware is doable. The big gotcha is that KVM needs to set the suppress #VE bit for all EPTEs when allocating a new MMU page, otherwise not-present faults on zero-initialized EPTEs will get reflected.
Attached a patch that does the prep work in the MMU. The VMX usage would be:
kvm_mmu_set_spte_init_value(VMX_EPT_SUPPRESS_VE_BIT);
when EPT Violation #VEs are enabled. It's 64-bit only as it uses stosq to initialize EPTEs. 32-bit could also be supported by doing memcpy() from a static page.