linux-kernel - Re: [RFC PATCH v2 06/51] KVM: Query guest

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Message-ID: <e38f0573-520a-4fe8-91fc-797086ab5866@linux.intel.com>
Date: Wed, 28 May 2025 16:08:34 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>, Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH v2 06/51] KVM: Query guest_memfd for private/shared
 status



On 5/27/2025 11:55 AM, Yan Zhao wrote:
> On Wed, May 14, 2025 at 04:41:45PM -0700, Ackerley Tng wrote:
>> Query guest_memfd for private/shared status if those guest_memfds
>> track private/shared status.
>>
>> With this patch, Coco VMs can use guest_memfd for both shared and
>> private memory. If Coco VMs choose to use guest_memfd for both
>> shared and private memory, by creating guest_memfd with the
>> GUEST_MEMFD_FLAG_SUPPORT_SHARED flag, guest_memfd will be used to
>> provide the private/shared status of the memory, instead of
>> kvm->mem_attr_array.
>>
>> Change-Id: I8f23d7995c12242aa4e09ccf5ec19360e9c9ed83
>> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>> ---
>>   include/linux/kvm_host.h | 19 ++++++++++++-------
>>   virt/kvm/guest_memfd.c   | 22 ++++++++++++++++++++++
>>   2 files changed, 34 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index b317392453a5..91279e05e010 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -2508,12 +2508,22 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
>>   }
>>   
>>   #ifdef CONFIG_KVM_GMEM_SHARED_MEM
>> +
>>   bool kvm_gmem_memslot_supports_shared(const struct kvm_memory_slot *slot);
>> +bool kvm_gmem_is_private(struct kvm_memory_slot *slot, gfn_t gfn);
>> +
>>   #else
>> +
>>   static inline bool kvm_gmem_memslot_supports_shared(const struct kvm_memory_slot *slot)
>>   {
>>   	return false;
>>   }
>> +
>> +static inline bool kvm_gmem_is_private(struct kvm_memory_slot *slot, gfn_t gfn)
>> +{
>> +	return false;
>> +}
>> +
>>   #endif /* CONFIG_KVM_GMEM_SHARED_MEM */
>>   
>>   #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>> @@ -2544,13 +2554,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>>   		return false;
>>   
>>   	slot = gfn_to_memslot(kvm, gfn);
>> -	if (kvm_slot_has_gmem(slot) && kvm_gmem_memslot_supports_shared(slot)) {
>> -		/*
>> -		 * For now, memslots only support in-place shared memory if the
>> -		 * host is allowed to mmap memory (i.e., non-Coco VMs).
>> -		 */
>> -		return false;
>> -	}
>> +	if (kvm_slot_has_gmem(slot) && kvm_gmem_memslot_supports_shared(slot))
>> +		return kvm_gmem_is_private(slot, gfn);
> When userspace gets an exit reason KVM_EXIT_MEMORY_FAULT, looks it needs to
> update both KVM memory attribute and gmem shareability, via two separate ioctls?
IIUC, when userspace sets flag GUEST_MEMFD_FLAG_SUPPORT_SHARED to create the
guest_memfd, the check for memory attribute will go through the guest_memfd way,
the information in kvm->mem_attr_array will not be used.

So if userspace sets GUEST_MEMFD_FLAG_SUPPORT_SHARED, it uses
KVM_GMEM_CONVERT_SHARED/PRIVATE to update gmem shareability.
If userspace doesn't set GUEST_MEMFD_FLAG_SUPPORT_SHARED, it still uses
KVM_SET_MEMORY_ATTRIBUTES to update KVM memory attribute tracking.


>
>
>>   	return kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
>>   }