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

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Date: Wed, 27 Mar 2024 08:47:54 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Isaku Yamahata <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>, Kai Huang <kai.huang@...el.com>,
 chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
 isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 048/130] KVM: Allow page-sized MMU caches to be
 initialized with custom 64-bit values



On 3/27/2024 1:34 AM, Isaku Yamahata wrote:
> On Tue, Mar 26, 2024 at 11:53:02PM +0800,
> Binbin Wu <binbin.wu@...ux.intel.com> wrote:
>
>>
>> On 2/26/2024 4:25 PM, 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>
>>> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.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?
> I don't see much value.  Is your concern sizeof() part?
> If so, we can replace it with 8.
>
>          memset64(page, mc->init_value, PAGE_SIZE / 8);

Yes, but it's trivial. So, up to you. :)