Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE hypercall

[Jeremi Piotrowski]

On Fri, Jan 27, 2023 at 08:35:58AM -0800, Jeremi Piotrowski wrote:
> On Wed, Dec 14, 2022 at 01:39:59PM -0600, Michael Roth wrote:
> > From: Nikunj A Dadhania <nikunj@xxxxxxx>
> > 
> > KVM_HC_MAP_GPA_RANGE hypercall is used by the SEV guest to notify a
> > change in the page encryption status to the hypervisor.
> > 
> > The hypercall exits to userspace with KVM_EXIT_HYPERCALL exit code,
> > currently this is used for explicit memory conversion between
> > shared/private for memfd based private memory.
> > 
> > Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> > Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c  | 8 ++++++++
> >  virt/kvm/kvm_main.c | 1 +
> >  2 files changed, 9 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index bb6adb216054..732f9cbbadb5 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9649,6 +9649,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> 
> Couldn't find a better commit to comment on:
> when the guest has the ptp-kvm module, it will issue a KVM_HC_CLOCK_PAIRING
> hypercall. This will pass sev_es_validate_vmgexit validation and end up in this
> function where kvm_pv_clock_pairing() is called, and that calls
> kvm_write_guest(). This results in a CPU soft-lockup, at least in my testing.
> 
> Are there any emulated hypercalls that make sense for snp guests? We should
> block at least the ones that definitely don't work.
> 
> Jeremi

So turns out the soft-lockup is a nested issue (details here for those
interested: [^1]), but the questions still stands, of whether we should
block kvm_write_page (and similar) explicitly or rely on the rmp fault.

[^1]: https://github.com/jepio/linux/commit/6c3bdf552e93664ae172660e24ceceed60fd4df5