linux-kernel - Re: [PATCH v13 030/113] KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

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Date:   Wed, 5 Apr 2023 10:11:21 +0300
From:   Zhi Wang <zhi.wang.linux@...il.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH v13 030/113] KVM: Allow page-sized MMU caches to be
 initialized with custom 64-bit values

On Sun, 12 Mar 2023 10:55:54 -0700
isaku.yamahata@...el.com wrote:

> From: Sean Christopherson <seanjc@...gle.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@...gle.com>

Your Sighned-off-by is required as well.

> ---
>  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 2728d49bbdf6..7c2b9332b7c5 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 f8495e27d210..87400796df6e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -381,12 +381,17 @@ 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)
> @@ -401,6 +406,13 @@ 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;