linux-kernel - Re: [PATCH 07/13] KVM: x86/mmu: Track PRIVATE impact on hugepage mappings for all memslots

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Date:   Wed, 27 Sep 2023 07:25:21 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [PATCH 07/13] KVM: x86/mmu: Track PRIVATE impact on hugepage
 mappings for all memslots

On Wed, Sep 27, 2023, Binbin Wu wrote:
> 
> On 9/22/2023 4:33 AM, Sean Christopherson wrote:
> > Track the effects of private attributes on potential hugepage mappings if
> > the VM supports private memory, i.e. even if the target memslot can only
> > ever be mapped shared.  If userspace configures a chunk of memory as
> > private, KVM must not allow that memory to be mapped shared regardless of
> > whether or not the *current* memslot can be mapped private.  E.g. if the
> > guest accesses a private range using a shared memslot, then KVM must exit
> > to userspace.
> How does usersapce handle this case?
> IIRC, in gmem v12 patch set, it says a memslot can not be convert to private
> from shared.
> So, userspace should delete the old memslot and and a new one?

That depends on the contract between userspace and the VM, e.g. if the access is
to a range that the VMM and the guest have agreed is shared-only, then the VMM
could also "resolve" the access by injecting an error or killing the VM.

But yes, deleting the memslot and creating a new one is the only approach that
will work if userspace wants to map the gfn as private.  I don't actually expect
any real world VMMs to actually do anything like this, the purpose of this change
is mainly to ensure KVM has robust, consistent behavior.