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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.09.16 at 19:29, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> 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?

Since you don't quote the specific commit(s), I would guess that
this was mainly an attempt by the author(s) to keep things simple
for themselves, i.e. not having to properly think through under
which conditions less than a full TLB flush would suffice.

Jan


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