linux-kernel - Re: [PATCH v11 030/113] KVM: x86/mmu: Replace hardcoded value 0 for the initial value for SPTE

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Message-ID: <Y9Fj/vgPEzfU1eof@google.com>
Date:   Wed, 25 Jan 2023 17:22:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zhi Wang <zhi.wang.linux@...il.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v11 030/113] KVM: x86/mmu: Replace hardcoded value 0 for
 the initial value for SPTE

On Wed, Jan 25, 2023, Zhi Wang wrote:
> On Thu, 12 Jan 2023 08:31:38 -0800
> isaku.yamahata@...el.com wrote:
> 
> This refactor patch is quite hacky.
> 
> Why not change the purpose of vcpu->arch.mmu_shadow_page.gfp_zero and let the
> callers respect that the initial value of spte can be configurable? It will be
> generic and not TDX-specific, then kvm_init_shadow_page() is not required,
> mmu_topup_shadow_page_cache() can be left un-touched as the refactor can cover
> other architectures.
> 
> 1) Let it store the expected nonpresent value and rename it to nonpresent_spte.


I agree that handling this in the common code would be cleaner, but repurposing
gfp_zero gets kludgy because it would require a magic value to say "don't initialize
the data", e.g. x86's mmu_shadowed_info_cache isn't pre-filled.

And supporting a custom 64-bit init value for kmem_cache-backed caches would require
restricting such caches to be a multiple of 8 bytes in size.

How about this?  Lightly tested.

From: Sean Christopherson <seanjc@...gle.com>
Date: Wed, 25 Jan 2023 16:55:01 +0000
Subject: [PATCH] KVM: Allow page-sized MMU caches to be initialized with
 custom 64-bit values

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@...gle.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 76de36e56cdf..67972db17b55 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -94,6 +94,7 @@ struct kvm_mmu_memory_cache {
 	int nobjs;
 	gfp_t gfp_zero;
 	gfp_t gfp_custom;
+	u64 init_value;
 	struct kmem_cache *kmem_cache;
 	int capacity;
 	void **objects;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d255964ec331..78f1e49179a7 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -380,12 +380,17 @@ 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));
+	return page;
 }
 
 int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
@@ -400,6 +405,13 @@ 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;

base-commit: 503f0315c97739d3f8e645c500d81757dfbf76be
--