On Wed, Aug 16, 2023, Sean Christopherson wrote: > On Tue, Aug 01, 2023, Mingwei Zhang wrote: > > Add the description of tdp_mmu_root_count into kvm_mmu_page description and > > combine it with the description of root_count. tdp_mmu_root_count is an > > atomic counter used only in TDP MMU. Update the doc. > > > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > > Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx> > > --- > > Documentation/virt/kvm/x86/mmu.rst | 12 ++++++++---- > > 1 file changed, 8 insertions(+), 4 deletions(-) > > > > diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst > > index 17d90974204e..40daf8beb9b1 100644 > > --- a/Documentation/virt/kvm/x86/mmu.rst > > +++ b/Documentation/virt/kvm/x86/mmu.rst > > @@ -229,10 +229,14 @@ Shadow pages contain the following information: > > can be calculated from the gfn field when used. In addition, when > > role.direct is set, KVM does not track access permission for each of the > > gfn. See role.direct and gfn. > > - root_count: > > - A counter keeping track of how many hardware registers (guest cr3 or > > - pdptrs) are now pointing at the page. While this counter is nonzero, the > > - page cannot be destroyed. See role.invalid. > > + root_count / tdp_mmu_root_count: > > + root_count is a reference counter for root shadow pages in Shadow MMU. > > + vCPUs elevate the refcount when getting a shadow page that will be used as > > + a root page, i.e. page that will be loaded into hardware directly (CR3, > > + PDPTRs, nCR3 EPTP). Root pages cannot be destroyed while their refcount is > > + non-zero. See role.invalid. tdp_mmu_root_count is similar but exclusively > > + used in TDP MMU as an atomic refcount. When the value is non-zero, it > > + allows vCPUs acquire references while holding mmu_lock for read. > > That last sentence is wrong. *vCPUs* can't acquire references while holding > mmu_lock for read. And actually, they don't ever put references while holding > for read either. vCPUs *must* hold mmu_lock for write to obtain a new root, > Not putting references while holding mmu_lock for read is mostly an implementation > quirk. > > Maybe replace it with this? > > tdp_mmu_root_count is similar but exclusively used in the TDP MMU as an > atomic refcount (select TDP MMU flows walk all roots while holding mmu_lock > for read, e.g. when clearing dirty bits). hmm, I think all the content within the bracket is details and we should not mention them at all. In fact, when I see the implementation, the last refcount of tdp_mmu_root_count is treated differently. Those details should be instead mentioned in code or comments instead of documentation as they may evolve much faster. So, I will remove the last sentence.
