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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.10.16 at 17:41, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> There are a ton of calls to flush_area_local, and a good chunk of them
> with the idle vCPU being the active one when it is called. As for
> write_cr3, there are also a lot of calls there. When I added some
> debug output to observe just how many dom0 would take almost an hour
> to boot and the serial line would just be spammed with that printk. So
> even if there no HVM paths leading there, others paths definitely do
> that affect HVM guests by making all of them take on a new tag next
> time they are scheduled.

Well, that's all fine, but - considering what Tim explained in great
detail - not really relevant. We just can't blindly eliminate those
safety flushes. What we can eliminate are just flushes where we
know they're not safety ones, i.e. such initiated by guest CR
updates (or alike), and I'm afraid there aren't that many.

For the safety flushes the best we may be able to do would appear
to be to limit their scope: If we knew which domains can possibly
have active mappings, we could avoid flushing unrelated ASIDs. But
even then we'd have to flush full address spaces, as we don't know
at which _virtual_ address(es) such mappings may have lived (and
there are no mechanisms to flush based on guest or host physical
address).

Jan


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