[Xen-devel] Question about VPID during MOV-TO-CR3

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

Hi all,
I'm trying to figure out the design decision regarding the handling of
guest MOV-TO-CR3 operations and TLB flushes. AFAICT since support for
VPID has been added to Xen, every guest MOV-TO-CR3 flushes the TLB
(vmx_cr_access -> hvm_mov_to_cr -> hvm_set_cr3 -> paging_update_cr3 ->
hap_update_cr3 -> vmx_update_guest_cr -> hvm_asid_flush_vcpu). From a
TLB utilization point-of-view this seems to be rather wasteful.
Furthermore, it even breaks the guests' ability to take advantage of
PCID, as the TLB just guts flushed when a new process is scheduled.
Does anyone have an insight into what was the rationale behind this?

Thanks,
Tamas

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