On Thu, Mar 21, 2024 at 02:24:12PM -0700, Isaku Yamahata wrote: >On Thu, Mar 21, 2024 at 12:11:11AM +0000, >"Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote: > >> On Mon, 2024-02-26 at 00:25 -0800, isaku.yamahata@xxxxxxxxx wrote: >> > To handle private page tables, argument of is_private needs to be >> > passed >> > down. Given that already page level is passed down, it would be >> > cumbersome >> > to add one more parameter about sp. Instead replace the level >> > argument with >> > union kvm_mmu_page_role. Thus the number of argument won't be >> > increased >> > and more info about sp can be passed down. >> > >> > For private sp, secure page table will be also allocated in addition >> > to >> > struct kvm_mmu_page and page table (spt member). The allocation >> > functions >> > (tdp_mmu_alloc_sp() and __tdp_mmu_alloc_sp_for_split()) need to know >> > if the >> > allocation is for the conventional page table or private page table. >> > Pass >> > union kvm_mmu_role to those functions and initialize role member of >> > struct >> > kvm_mmu_page. >> >> tdp_mmu_alloc_sp() is only called in two places. One for the root, and >> one for the mid-level tables. >> >> In later patches when the kvm_mmu_alloc_private_spt() part is added, >> the root case doesn't need anything done. So the code has to take >> special care in tdp_mmu_alloc_sp() to avoid doing anything for the >> root. >> >> It only needs to do the special private spt allocation in non-root >> case. If we open code that case, I think maybe we could drop this >> patch, like the below. >> >> The benefits are to drop this patch (which looks to already be part of >> Paolo's series), and simplify "KVM: x86/mmu: Add a private pointer to >> struct kvm_mmu_page". I'm not sure though, what do you think? Only >> build tested. > >Makes sense. Until v18, it had config to disable private mmu part at >compile time. Those functions have #ifdef in mmu_internal.h. v19 >dropped the config for the feedback. > https://lore.kernel.org/kvm/Zcrarct88veirZx7@xxxxxxxxxx/ > >After looking at mmu_internal.h, I think the following three function could be >open coded. >kvm_mmu_private_spt(), kvm_mmu_init_private_spt(), kvm_mmu_alloc_private_spt(), >and kvm_mmu_free_private_spt(). It took me a few minutes to figure out why the mirror root page doesn't need a private_spt. Per TDX module spec: Secure EPT’s root page (EPML4 or EPML5, depending on whether the host VMM uses 4-level or 5-level EPT) does not need to be explicitly added. It is created during TD initialization (TDH.MNG.INIT) and is stored as part of TDCS. I suggest adding the above as a comment somewhere even if we decide to open-code kvm_mmu_alloc_private_spt(). IMO, some TDX details bleed into KVM MMU regardless of whether we open-code kvm_mmu_alloc_private_spt() or not. This isn't good though I cannot think of a better solution.
