Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIST ioctl

[Ashish Kalra]

On Mon, Mar 08, 2021 at 11:51:57AM -0800, Sean Christopherson wrote:
> On Mon, Mar 08, 2021, Ashish Kalra wrote:
> > 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@xxxxxxx> 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.
> 
> Relying on the RMP table itself would require locking the RMP table for an
> extended duration, and walking the entire RMP to find shared pages would be
> very inefficient.
> 
> > As KVM maintains the RMP table, therefore we will need SET/GET type of
> > interfaces to provide the guest page encryption status to userspace.
> 
> Hrm, somehow I temporarily forgot about SNP and TDX adding their own hypercalls
> for converting between shared and private.  And in the case of TDX, the hypercall
> can't be trusted, i.e. is just a hint, otherwise the guest could induce a #MC in
> the host.

One question here, is this because if hypercall can cause direct
modifications to the shared EPT, it can induce #MC in the host ?

Thanks,
Ashish