linux-kernel - Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES

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Date:   Mon, 8 Mar 2021 10:40:14 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Steve Rutherford <srutherford@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "venu.busireddy@...cle.com" <venu.busireddy@...cle.com>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        Will Deacon <will@...nel.org>,
        Quentin Perret <qperret@...gle.com>
Subject: Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIST
 ioctl

On Fri, Feb 26, 2021 at 09:44:41AM -0800, Sean Christopherson wrote:
> +Will and Quentin (arm64)
> 
> Moving the non-KVM x86 folks to bcc, I don't they care about KVM details at this
> point.
> 
> On Fri, Feb 26, 2021, Ashish Kalra wrote:
> > On Thu, Feb 25, 2021 at 02:59:27PM -0800, Steve Rutherford wrote:
> > > On Thu, Feb 25, 2021 at 12:20 PM Ashish Kalra <ashish.kalra@....com> wrote:
> > > Thanks for grabbing the data!
> > > 
> > > I am fine with both paths. Sean has stated an explicit desire for
> > > hypercall exiting, so I think that would be the current consensus.
> 
> Yep, though it'd be good to get Paolo's input, too.
> 
> > > If we want to do hypercall exiting, this should be in a follow-up
> > > series where we implement something more generic, e.g. a hypercall
> > > exiting bitmap or hypercall exit list. If we are taking the hypercall
> > > exit route, we can drop the kvm side of the hypercall.
> 
> I don't think this is a good candidate for arbitrary hypercall interception.  Or
> rather, I think hypercall interception should be an orthogonal implementation.
> 
> The guest, including guest firmware, needs to be aware that the hypercall is
> supported, and the ABI needs to be well-defined.  Relying on userspace VMMs to
> implement a common ABI is an unnecessary risk.
> 
> We could make KVM's default behavior be a nop, i.e. have KVM enforce the ABI but
> require further VMM intervention.  But, I just don't see the point, it would
> save only a few lines of code.  It would also limit what KVM could do in the
> future, e.g. if KVM wanted to do its own bookkeeping _and_ exit to userspace,
> then mandatory interception would essentially make it impossible for KVM to do
> bookkeeping while still honoring the interception request.
> 
> However, I do think it would make sense to have the userspace exit be a generic
> exit type.  But hey, we already have the necessary ABI defined for that!  It's
> just not used anywhere.
> 
> 	/* KVM_EXIT_HYPERCALL */
> 	struct {
> 		__u64 nr;
> 		__u64 args[6];
> 		__u64 ret;
> 		__u32 longmode;
> 		__u32 pad;
> 	} hypercall;
> 
> 
> > > Userspace could also handle the MSR using MSR filters (would need to
> > > confirm that).  Then userspace could also be in control of the cpuid bit.
> 
> An MSR is not a great fit; it's x86 specific and limited to 64 bits of data.
> The data limitation could be fudged by shoving data into non-standard GPRs, but
> that will result in truly heinous guest code, and extensibility issues.
> 
> The data limitation is a moot point, because the x86-only thing is a deal
> breaker.  arm64's pKVM work has a near-identical use case for a guest to share
> memory with a host.  I can't think of a clever way to avoid having to support
> TDX's and SNP's hypervisor-agnostic variants, but we can at least not have
> multiple KVM variants.
> 

Potentially, there is another reason for in-kernel hypercall handling
considering SEV-SNP. In case of SEV-SNP the RMP table tracks the state
of each guest page, for instance pages in hypervisor state, i.e., pages
with C=0 and pages in guest valid state with C=1.

Now, there shouldn't be a need for page encryption status hypercalls on 
SEV-SNP as KVM can track & reference guest page status directly using 
the RMP table.

As KVM maintains the RMP table, therefore we will need SET/GET type of
interfaces to provide the guest page encryption status to userspace.

For the above reason if we do in-kernel hypercall handling for page
encryption status (which we probably won't require for SEV-SNP &
correspondingly there will be no hypercall exiting), then we can
implement a standard GET/SET ioctl interface to get/set the guest page
encryption status for userspace, which will work across SEV, SEV-ES and
SEV-SNP.

Thanks,
Ashish