On Tue, Jul 12, 2022 at 10:22:50AM -0700, Isaku Yamahata wrote: > On Tue, Jul 12, 2022 at 06:54:19PM +0800, > Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote: > > > On Tue, Jul 12, 2022 at 01:07:20PM +0800, Chao Gao wrote: > > > On Mon, Jul 11, 2022 at 08:17:01AM -0700, Isaku Yamahata wrote: > > > >Hi. Because my description on large page support was terse, I wrote up more > > > >detailed one. Any feedback/thoughts on large page support? > > > > > > > >TDP MMU large page support design > > > > > > > >Two main discussion points > > > >* how to track page status. private vs shared, no-largepage vs can-be-largepage > > > > > > ... > > > > > > > > > > >Tracking private/shared and large page mappable > > > >----------------------------------------------- > > > >VMM needs to track that page is mapped as private or shared at 4KB granularity. > > > >For efficiency of EPT violation path (****), at 2MB and 1GB level, VMM should > > > >track the page can be mapped as a large page (regarding private/shared). VMM > > > >updates it on MapGPA and references it on the EPT violation path. (****) > > > > > > Isaku, > > > > > > + Peng Chao > > > > > > Doesn't UPM guarantee that 2MB/1GB large page in CR3 should be either all > > > private or all shared? > > > > > > KVM always retrieves the mapping level in CR3 and enforces that EPT's > > > page level is not greater than that in CR3. My point is if UPM already enforces > > > no mixed pages in a large page, then KVM needn't do that again (UPM can > > > be trusted). > > > > The backing store in the UMP can tell KVM which page level it can > > support for a given private gpa, similar to host_pfn_mapping_level() for > > shared address. > > > > However, this solely represents the backing store's capability, KVM > > still needs additional info to decide whether that can be safely mapped > > as 2M/1G, e.g. all the following pages in the 2M/1G range should be all > > private, currently this is not something backing store can tell. > > This argument applies to shared GPA. The shared pages is backed by normal file > mapping with UPM. When KVM is mapping shared GPA, the same check is needed. So > I think KVM has to track all private or all shared or no-largepage at 2MB/1GB > level. If UPM tracks shared-or-private at 4KB level, probably KVM may not need to > track it at 4KB level. Right, the same also applies to shared memory. All the info we need is whether pages of a 2M range is all private/shared but not mixed. UPM v7 has code tracking that in KVM and previously versions we track that in the backing store which has been discussed not a good idea. Chao > > > > Actually, in UPM v7 we let KVM record this info so one possible solution > > is making use of it. > > > > https://lkml.org/lkml/2022/7/6/259 > > > > Then to map a page as 2M, KVM needs to check: > > - Memory backing store support that level > > - All pages in 2M range are private as we recorded through > > KVM_MEMORY_ENCRYPT_{UN,}REG_REGION > > - No existing partial 4K map(s) in 2M range > -- > Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
