Re: [PATCH v3 5/6] KVM: Documentation: Add the missing description for mmu_valid_gen into kvm_mmu_page

[Mingwei Zhang]

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      page is obsolete, i.e. belongs to a previous MMU generation.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'.