linux-kernel - Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA

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Message-ID: <Y8bYhpITRA+RZzWK@google.com>
Date:   Tue, 17 Jan 2023 17:19:02 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Nikunj A. Dadhania" <nikunj@....com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
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        tony.luck@...el.com, marcorr@...gle.com,
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        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        harald@...fian.com
Subject: Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE
 hypercall

On Mon, Jan 16, 2023, Nikunj A. Dadhania wrote:
> On 13/01/23 21:47, Sean Christopherson wrote:
> > It's perfectly legal for userspace to create the private memslot in response
> > to a guest request.
> 
> Sean, did not understand this part, how could a memslot be created on a guest request?

KVM_HC_MAP_GPA_RANGE gets routed to host userspace, at that point userspace can
take any action it wants to satisfy the guest request.  E.g. a userspace+guest
setup could define memory as shared by default, and only create KVM_MEM_PRIVATE
memslots for memory that the guest explicitly requests to be mapped private.

I don't anticipate any real world use cases actually doing something like that,
but I also don't see any value in going out of our way to disallow it.  Normally
I like to be conservative when it comes to KVM's uAPI, e.g. allow the minimum
needed to support known use cases, but restricting KVM_HC_MAP_GPA_RANGE doesn't
actually achieve anything and just makes things more complex for KVM.  E.g. the
behavior is non-deterministic from KVM's perspective if a userspace memslots update
is in-progress.