On Wed, Dec 14, 2022 at 01:39:59PM -0600, Michael Roth wrote:
> From: Nikunj A Dadhania <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Michael Roth <[email protected]>
> ---
> 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
> break;
> case KVM_HC_MAP_GPA_RANGE: {
> u64 gpa = a0, npages = a1, attrs = a2;
> + struct kvm_memory_slot *slot;
>
> ret = -KVM_ENOSYS;
> if (!(vcpu->kvm->arch.hypercall_exit_enabled & (1 << KVM_HC_MAP_GPA_RANGE)))
> @@ -9660,6 +9661,13 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> break;
> }
>
> + slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
> + if (!vcpu->kvm->arch.upm_mode ||
> + !kvm_slot_can_be_private(slot)) {
> + ret = 0;
> + break;
> + }
> +
> vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
> vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
> vcpu->run->hypercall.args[0] = gpa;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d2daa049e94a..73bf0bdedb59 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2646,6 +2646,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>
> return NULL;
> }
> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>
> bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
> {
> --
> 2.25.1
>
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 <[email protected]>
> >
> > 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 <[email protected]>
> > Signed-off-by: Michael Roth <[email protected]>
> > ---
> > 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