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Date:   Thu, 6 Oct 2022 17:58:00 +0300
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Chao Peng <chao.p.peng@linux.intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based
 private memory
Message-ID: <Yz7s+JIexAHJm5dc@kernel.org>
References: <20220915142913.2213336-1-chao.p.peng@linux.intel.com>
 <20220915142913.2213336-3-chao.p.peng@linux.intel.com>
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On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote:
> This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> additional KVM memslot fields private_fd/private_offset to allow
> userspace to specify that guest private memory provided from the
> private_fd and guest_phys_addr mapped at the private_offset of the
> private_fd, spanning a range of memory_size.
> 
> The extended memslot can still have the userspace_addr(hva). When use, a
> single memslot can maintain both private memory through private
> fd(private_fd/private_offset) and shared memory through
> hva(userspace_addr). Whether the private or shared part is visible to
> guest is maintained by other KVM code.

What is anyway the appeal of private_offset field, instead of having just
1:1 association between regions and files, i.e. one memfd per region?

If this was the case, then an extended struct would not be needed in the
first place. A simple union inside the existing struct would do:

        union {
                __u64 userspace_addr,
                __u64 private_fd,
        };

BR, Jarkko