On 22.09.2022 18:05, Roger Pau Monne wrote: > memory_type_changed() is currently only implemented for Intel EPT, and > results in the invalidation of EMT attributes on all the entries in > the EPT page tables. Such invalidation causes EPT_MISCONFIG vmexits > when the guest tries to access any gfns for the first time, which > results in the recalculation of the EMT for the accessed page. The > vmexit and the recalculations are expensive, and as such should be > avoided when possible. > > Remove the call to memory_type_changed() from > XEN_DOMCTL_memory_mapping: there are no modifications of the > iomem_caps ranges anymore that could alter the return of > cache_flush_permitted() from that domctl.
I certainly agree - this was an oversight when the two aspects were split. One might argue this is a (performance) fix to the earlier commit, and hence might want to go on its own with a Fixes: tag. > Calls to memory_type_changed() resulting from changes to the domain > iomem_caps or ioport_caps ranges are only relevant for EMT > calculations if the IOMMU is not enabled, and the call has resulted in > a change to the return value of cache_flush_permitted(). I'm less certain here: These shouldn't be frequent operations, so their impact on the guest should be limited? And if we were to restrict the calls, I think we need to clearly tie together the various places which need updating together in case e.g. the condition in epte_get_entry_emt() is changed. Minimally by way of comments, but maybe by way of a small helper function (for which I can't seem to be able to think of a good name) sitting next to epte_get_entry_emt(). > Signed-off-by: Roger Pau Monné <roger....@citrix.com> > --- > I feel it's a bit weird to have calls to memory_type_changed() in > common domctl code - for once the domctl that trigger the call doesn't > change memory types, just adds or removes ranges from iomem_caps > (which in turn affects the behaviour of epte_get_entry_emt()). Do you have a better suggestion? Jan
