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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 21 Oct 2020 11:20:56 -0700
Message-ID: <CALCETrXn_ghtLK34jmKSSp5_SF6hh5GOfBLKdxXgp5ZTbN8uEA@mail.gmail.com>
Subject: Re: [RFCv2 00/16] KVM protected memory extension
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Will Drewry <wad@chromium.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        "Kleen, Andi" <andi.kleen@intel.com>,
        Liran Alon <liran.alon@oracle.com>,
        Mike Rapoport <rppt@kernel.org>, X86 ML <x86@kernel.org>,
        kvm list <kvm@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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> On Oct 19, 2020, at 11:19 PM, Kirill A. Shutemov <kirill@shutemov.name> wrote:

> For removing the userspace mapping, use a trick similar to what NUMA
> balancing does: convert memory that belongs to KVM memory slots to
> PROT_NONE: all existing entries converted to PROT_NONE with mprotect() and
> the newly faulted in pages get PROT_NONE from the updated vm_page_prot.
> The new VMA flag -- VM_KVM_PROTECTED -- indicates that the pages in the
> VMA must be treated in a special way in the GUP and fault paths. The flag
> allows GUP to return the page even though it is mapped with PROT_NONE, but
> only if the new GUP flag -- FOLL_KVM -- is specified. Any userspace access
> to the memory would result in SIGBUS. Any GUP access without FOLL_KVM
> would result in -EFAULT.
>

I definitely like the direction this patchset is going in, and I think
that allowing KVM guests to have memory that is inaccessible to QEMU
is a great idea.

I do wonder, though: do we really want to do this with these PROT_NONE
tricks, or should we actually come up with a way to have KVM guest map
memory that isn't mapped into QEMU's mm_struct at all?  As an example
of the latter, I mean something a bit like this:

https://lkml.kernel.org/r/CALCETrUSUp_7svg8EHNTk3nQ0x9sdzMCU=h8G-Sy6=SODq5GHg@mail.gmail.com

I don't mean to say that this is a requirement of any kind of
protected memory like this, but I do think we should understand the
tradeoffs, in terms of what a full implementation looks like, the
effort and time frames involved, and the maintenance burden of
supporting whatever gets merged going forward.