Re: [RFC PATCH v5 44/45] KVM: x86/mmu: Add support for splitting S-EPT hugepages on conversion

[Yan Zhao]

On Wed, Jan 28, 2026 at 05:15:16PM -0800, Sean Christopherson wrote:
> Add support for splitting S-EPT hugepages in preparation for converting a
> subset of a hugepage to be shared, as KVM must precisely zap/remove S-EPT
> entries to avoid clobbering guest memory (the lifetime of guest private
> memory is tied to the S-EPT).  I.e. KVM needs to first split a hugepage so
> that only the to-be-converted small pages can be zapped.
> 
> To avoid unnecessary work, e.g. if only the tail/end page of massive region
> isn't aligned to the conversion, explicitly detect unaligned head and tail
> pages relative to the max page size support by KVM, i.e. head/tail pages
> that will undergo partial conversion.
> 
> To support splitting an S-EPT hugepage without a vCPU, add a per-VM PAMT
> cache, along with a mutex to guard the cache.  Using a mutex, e.g. versus
> a spinlock, is important at it allows KVM to allocate memory *without*
> dropping the lock, i.e. so that the PAMT cache can be topped-up as needed
> without needed to juggle arch.tdp_mmu_external_cache_lock.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  8 +++-
>  arch/x86/kvm/mmu/mmu.c          |  2 +-
>  arch/x86/kvm/mmu/tdp_mmu.c      | 72 +++++++++++++++++++++++++++++++--
>  arch/x86/kvm/vmx/tdx.c          | 34 +++++++++++++---
>  arch/x86/kvm/vmx/tdx.h          |  2 +
>  5 files changed, 107 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 385f1cf32d70..54dea90a53dc 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1563,6 +1563,12 @@ struct kvm_arch {
>  	 * the code to do so.
>  	 */
>  	spinlock_t tdp_mmu_pages_lock;
> +
> +	/*
> +	 * Protect the per-VM cache of pre-allocate pages used to populate the
> +	 * Dynamic PAMT when splitting S-EPT huge pages without a vCPU.
> +	 */
> +	struct mutex tdp_mmu_external_cache_lock;
Missing "spin_lock_init(&kvm->arch.tdp_mmu_external_cache_lock);" in
kvm_mmu_init_tdp_mmu().

Will check the patch you replied next week.

>  #endif /* CONFIG_X86_64 */