On Wed, Aug 16, 2023, Sean Christopherson wrote: > On Tue, Aug 01, 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 | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst > > index 40daf8beb9b1..581e53fa00a2 100644 > > --- a/Documentation/virt/kvm/x86/mmu.rst > > +++ b/Documentation/virt/kvm/x86/mmu.rst > > @@ -208,6 +208,16 @@ 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: > > + The MMU generation of this page, used to fast zap of all MMU pages within a > > + VM without blocking vCPUs. > > KVM still blocks vCPUs, just for far less time. How about this? > > The MMU generation of this page, used to determine whether or not a shadow > page is obsolete, i.e. belongs to a previous MMU generation. KVM changes > the MMU generation when all shadow pages need to be invalidated, e.g. if a > memslot is deleted, and so effectively marks all shadow pages as obsolete > without having to touch each page. Marking shadow pages obsolete allows > KVM to zap them in the background, i.e. so that vCPUs can run while the > zap is ongoing (using a root from the new generation). The MMU generation > is only ever '0' or '1' (slots_lock must be held until all pages from the > previous generation are zapped). > > Note, the TDP MMU... > Got you. I think instead of elaborating this, I would simply put this way: "... without blocking vCPUs for too long". The subsequent description basically tells how it works and naturally explains how it does not blocks vCPUs for too long. > > Specifically, KVM updates the per-VM valid MMU > > + generation which causes the mismatch of mmu_valid_gen for each mmu page. > > + This makes all existing MMU pages obsolete. Obsolete pages can't be used. > > + Therefore, vCPUs must load a new, valid root before re-entering the guest. > > + The MMU generation is only ever '0' or '1'.
