Re: [PATCH v4 08/11] KVM: guest_memfd: Add support for preservation via LUO

[tarunsahu@google.com]

Ackerley Tng <ackerleytng@google.com> writes:

> Tarun Sahu <tarunsahu@google.com> writes:
>
>>
>> [...snip...]
>>
>> +static bool kvm_gmem_luo_can_preserve(struct liveupdate_file_handler *handler, struct file *file)
>> +{
>> +	struct inode *inode = file_inode(file);
>> +	struct gmem_file *gmem_file;
>> +	struct kvm *kvm;
>> +
>> +	if (inode->i_sb->s_magic != GUEST_MEMFD_MAGIC)
>> +		return false;
>> +
>> +	gmem_file = file->private_data;
>> +	if (!gmem_file)
>> +		return false;
>> +
>> +	/*
>> +	 * Only Fully-shared guest_memfd preservation is supported
>> +	 */
>> +	if (!(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED))
>> +		return false;
>> +
>
> The above check can probably be removed. I think the core thing about
> this series is "we can preserve any memory that doesn't have any
> encryption state that goes along with it", and so the check for
> kvm_arch_has_private_mem() below would be sufficient to gate that.


Sounds Good. Will remove it in next version. I will rephrase the
Line: "Support only fully shared guest_memfd" => "Support only
guest_memfd that does not support private pages and also can not be
converted to private"

>
>> +	/*
>> +	 * It makes sure that no memory can be converted to private
>> +	 * even if it was initially fully shared (in-place conversions are
>> +	 * prevented).
>> +	 */
>> +	kvm = gmem_file->kvm;
>> +	if (kvm_arch_has_private_mem(kvm))
>> +		return false;
>> +
>
> For now, has_private_mem == has some encryption state that we can't
> preserve yet.
>
> This also gates off KVM_X86_SW_PROTECTED_VM, but I guess that is
> intended, since when work is done for preserving encryption state, we
> can then test using KVM_X86_SW_PROTECTED_VM, and only then, remove this
> check?

Yes, It will be enabled when private memory preservation will be supported.

>
>> +	if (mapping_large_folio_support(inode->i_mapping))
>> +		return false;
>> +
>> +	return true;
>> +}
>> +
>>
>> [...snip...]
>>