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

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On 2/26/2024 4:25 PM, isaku.yamahata@xxxxxxxxx wrote:
From: Sean Christopherson <seanjc@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
---
  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;





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