[Xen-devel] XSA-60 - how to get back to a sane state

["Jan Beulich" <JBeulich@xxxxxxxx>]

All,

Jinsong's patches having been in for nearly a month now, but not
being in a shape that would make releasing in 4.4 or backporting to
the older trees desirable, we need to come to a conclusion on
which way to go. Currently it looks like we have three options, but
of course I'll be happy to see other (better!) ones proposed.

1) Stay with what we have.

2) Revert 86d60e85 ("VMX: flush cache when vmentry back to UC
guest") in its entirety plus, perhaps, the change 62652c00 ("VMX:
fix cr0.cd handling") did to vmx_ctxt_switch_to().

3) Apply the attached patch that Andrew and I have been putting
together, with the caveat that it's still incomplete (see below).

The latter two are based on the observation that the amount of
cache flushing we do with what is in the master tree right now is
more than what we did prior to that patch series but still
insufficient. Hence the revert would get us back to the earlier
state (and obviously eliminate the performance problems that
were observed when doing too eager flushing), whereas
applying the extra 5th patch would get us closer to a proper
solution.

The problem with that new patch is it's - as said - still incomplete,
yet setting ->arch.hvm_vcpu.hypervisor_access_uc_hvm_memory
for all writes through map_domain_page_global() mappings would
get rather ugly:
- guest_walk_tables() (accumulating set_ad_bits() results)
- all writes through vcpu_info()
- event_fifo.c would need this scattered around
- hvm_load_segment_selector()'s setting of the accessed bit
- hvm_task_switch()'s handling of the busy bit
- all the nested HVM code's writes through ->nv_vvmcx

Plus the performance effects of it aren't really known yet (all we
do know is that the too eager "flush always prior to VM entry" is
causing unacceptable stalls).

Otoh we have had no reports that the lack of proper cache
flushing actually caused any problems, and hence reverting the
partial flushing introduced isn't likely to push new problems onto
people. Yet if, ever, such a problem would surface, it can be
expected to be very hard to diagnose.

Please, everyone having an opinion here - voice it.

Thanks, Jan

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