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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.10.16 at 16:12, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> yes, I understand that is the case when you do need to flush a guest.
> And yes, there seem to be paths that require to bump the tag of a
> specific guest for certain events (mov-to-cr4 with paging mode changes
> for example). What I'm poking at it here is that we invalidate the
> guest TLBs for _all_ guests very frequently. I can't find an
> explanation for why _that_ is required. AFAIK having the TLB tag
> guarantees that no other guest or Xen will have a chance to bump into
> stale entries given no guests or Xen share a TLB tag with each other.
> So the only time I see that we would have to flush all guest TLBs is
> when the tag overflows and we start from 1 again. What am I missing
> here?

Oh, I see - this indeed looks to be quite a bit more flushing than is
desirable. So the question, as you did put it already, is why it got
done that way in the first place. At the very least it would look like
more control would need to be given to the callers of both
write_cr3() and flush_area_local(). Tim?

Jan


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