On Tue, Jun 27, 2023, Sean Christopherson wrote: > On Mon, Jun 26, 2023, Mingwei Zhang wrote: > > Add the description for mmu_valid_gen into kvm_mmu_page description. > > mmu_valid_gen is used in shadow MMU for fast zapping. Update the doc to > > reflect that. > > > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > > Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx> > > --- > > Documentation/virt/kvm/x86/mmu.rst | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst > > index 97d695207e11..cc4bd190c93d 100644 > > --- a/Documentation/virt/kvm/x86/mmu.rst > > +++ b/Documentation/virt/kvm/x86/mmu.rst > > @@ -208,6 +208,10 @@ Shadow pages contain the following information: > > The page is not backed by a guest page table, but its first entry > > points to one. This is set if NPT uses 5-level page tables (host > > CR4.LA57=1) and is shadowing L1's 4-level NPT (L1 CR4.LA57=1). > > + mmu_valid_gen: > > + Used by comparing against kvm->arch.mmu_valid_gen to check whether the > > This needs to explain what the generation is, and where it comes from. > > The MMU generation of this page, used to effect a "fast" zap of all MMU pages > across all roots. To zap all pages in all roots without blocking vCPUs, e.g. > when deleting a memslot, KVM updates the per-VM valid MMU generation to mark > all existing pages and roots invalid/obsolete. Obsolete pages can't be used, > e.g. vCPUs must load a new, valid root before re-entering the guest. > > The MMU generation is only ever '0' or '1', as slots_lock must be held until > all obsolete pages are zapped and freed, i.e. there is exactly one valid > generation and (at most) one invalid generation. > > Note, the TDP MMU doesn't use mmu_gen as non-root TDP MMU pages are reachable > only from their owning root, whereas all pages for shadow MMUs are reachable > via the hash map. The TDP MMU uses role.invalid to track obsolete roots. Sean, thanks for the detailed explanation. I will pick the most of the content and get into the next version. > > And then big bonus points if you add > > Page Role > ========= > > to explain the purpose of the role, and how/when it's used in the shadow MMU versus > the TDP MMU. The shadow MMU's use of a hash map is a fundemental aspect that really > should be documented here. > > > + shadow page is obsolete thus a convenient variable for fast zapping. > > + Note that TDP MMU does not use mmu_valid_gen. > > gfn: > > Either the guest page table containing the translations shadowed by this > > page, or the base page frame for linear translations. See role.direct. > > -- > > 2.41.0.162.gfafddb0af9-goog > >
