Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

Nikunj A. Dadhania
2022-06-24T03:58:51Z

On 5/19/2022 9:07 PM, Chao Peng wrote:
> A page fault can carry the information of whether the access if private
> or not for KVM_MEM_PRIVATE memslot, this can be filled by architecture
> code(like TDX code). To handle page faut for such access, KVM maps the
> page only when this private property matches host's view on this page
> which can be decided by checking whether the corresponding page is
> populated in the private fd or not. A page is considered as private when
> the page is populated in the private fd, otherwise it's shared.
>
> For a successful match, private pfn is obtained with memfile_notifier
> callbacks from private fd and shared pfn is obtained with existing
> get_user_pages.
>
> For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
> userspace. Userspace then can convert memory between private/shared from
> host's view then retry the access.
>
> Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
> arch/x86/kvm/mmu.h | 1 +
> arch/x86/kvm/mmu/mmu.c | 70 +++++++++++++++++++++++++++++++--
> arch/x86/kvm/mmu/mmu_internal.h | 17 ++++++++
> arch/x86/kvm/mmu/mmutrace.h | 1 +
> arch/x86/kvm/mmu/paging_tmpl.h | 5 ++-
> include/linux/kvm_host.h | 22 +++++++++++
> 6 files changed, 112 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 7e258cc94152..c84835762249 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -176,6 +176,7 @@ struct kvm_page_fault {
>
> /* Derived from mmu and global state. */
> const bool is_tdp;
> + const bool is_private;
> const bool nx_huge_page_workaround_enabled;
>
> /*
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index afe18d70ece7..e18460e0d743 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2899,6 +2899,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
> if (max_level == PG_LEVEL_4K)
> return PG_LEVEL_4K;
>
> + if (kvm_slot_is_private(slot))
> + return max_level;

Can you explain the rationale behind the above change?
AFAIU, this overrides the transparent_hugepage=never setting for both
shared and private mappings.

> host_level = host_pfn_mapping_level(kvm, gfn, pfn, slot);
> return min(host_level, max_level);
> }

Regards
Nikunj