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Date:   Sat, 21 May 2022 21:03:01 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Sean Christopherson" <seanjc@google.com>
Cc:     "Chao Peng" <chao.p.peng@linux.intel.com>,
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Subject: Re: [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory
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On Fri, May 20, 2022, at 11:31 AM, Sean Christopherson wrote:

> But a dedicated KVM ioctl() to add/remove shared ranges would be easy 
> to implement
> and wouldn't necessarily even need to interact with the memslots.  It 
> could be a
> consumer of memslots, e.g. if we wanted to disallow registering regions 
> without an
> associated memslot, but I think we'd want to avoid even that because 
> things will
> get messy during memslot updates, e.g. if dirty logging is toggled or a 
> shared
> memory region is temporarily removed then we wouldn't want to destroy 
> the tracking.
>
> I don't think we'd want to use a bitmap, e.g. for a well-behaved guest, XArray
> should be far more efficient.
>
> One benefit to explicitly tracking this in KVM is that it might be 
> useful for
> software-only protected VMs, e.g. KVM could mark a region in the XArray 
> as "pending"
> based on guest hypercalls to share/unshare memory, and then complete 
> the transaction
> when userspace invokes the ioctl() to complete the share/unshare.

That makes sense.

If KVM goes this route, perhaps there the allowed states for a GPA should include private, shared, and also private-and-shared.  Then anyone who wanted to use the same masked GPA for shared and private on TDX could do so if they wanted to.