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).
