Re: [PATCH v19 048/130] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

Binbin Wu
2024-03-26T15:53:10Z

On 2/26/2024 4:25 PM, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <seanjc@google.com>
>
> Add support to MMU caches for initializing a page with a custom 64-bit
> value, e.g. to pre-fill an entire page table with non-zero PTE values.
> The functionality will be used by x86 to support Intel's TDX, which needs
> to set bit 63 in all non-present PTEs in order to prevent !PRESENT page
> faults from getting reflected into the guest (Intel's EPT Violation #VE
> architecture made the less than brilliant decision of having the per-PTE
> behavior be opt-out instead of opt-in).
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> include/linux/kvm_types.h | 1 +
> virt/kvm/kvm_main.c | 16 ++++++++++++++--
> 2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index 9d1f7835d8c1..60c8d5c9eab9 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -94,6 +94,7 @@ struct gfn_to_pfn_cache {
> struct kvm_mmu_memory_cache {
> gfp_t gfp_zero;
> gfp_t gfp_custom;
> + u64 init_value;
> struct kmem_cache *kmem_cache;
> int capacity;
> int nobjs;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index de38f308738e..d399009ef1d7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -401,12 +401,17 @@ static void kvm_flush_shadow_all(struct kvm *kvm)
> static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
> gfp_t gfp_flags)
> {
> + void *page;
> +
> gfp_flags |= mc->gfp_zero;
>
> if (mc->kmem_cache)
> return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
> - else
> - return (void *)__get_free_page(gfp_flags);
> +
> + page = (void *)__get_free_page(gfp_flags);
> + if (page && mc->init_value)
> + memset64(page, mc->init_value, PAGE_SIZE / sizeof(mc->init_value));

Do we need a static_assert() to make sure mc->init_value is 64bit?

> + return page;
> }
>
> int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
> @@ -421,6 +426,13 @@ int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity,
> if (WARN_ON_ONCE(!capacity))
> return -EIO;
>
> + /*
> + * Custom init values can be used only for page allocations,
> + * and obviously conflict with __GFP_ZERO.
> + */
> + if (WARN_ON_ONCE(mc->init_value && (mc->kmem_cache || mc->gfp_zero)))
> + return -EIO;
> +
> mc->objects = kvmalloc_array(sizeof(void *), capacity, gfp);
> if (!mc->objects)
> return -ENOMEM;