Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event

[Alexey Kardashevskiy]

On 20/10/23 01:57, Sean Christopherson wrote:
> On Thu, Oct 19, 2023, Alexey Kardashevskiy wrote:
> > On 19/10/23 00:48, Sean Christopherson wrote:
> > > static int snp_handle_ext_guest_request(struct vcpu_svm *svm)
> > > {
> > > 	struct kvm_vcpu *vcpu = &svm->vcpu;
> > > 	struct kvm *kvm = vcpu->kvm;
> > > 	struct kvm_sev_info *sev;
> > > 	unsigned long exitcode;
> > > 	u64 data_gpa;
> > >
> > > 	if (!sev_snp_guest(vcpu->kvm)) {
> > > 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_GUEST);
> > > 		return 1;
> > > 	}
> > >
> > > 	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
> > > 	if (!IS_ALIGNED(data_gpa, PAGE_SIZE)) {
> > > 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_ADDRESS);
> > > 		return 1;
> > > 	}
> > >
> > > 	vcpu->run->hypercall.nr		 = KVM_HC_SNP_GET_CERTS;
> > > 	vcpu->run->hypercall.args[0]	 = data_gpa;
> > > 	vcpu->run->hypercall.args[1]	 = vcpu->arch.regs[VCPU_REGS_RBX];
> > > 	vcpu->run->hypercall.flags	 = KVM_EXIT_HYPERCALL_LONG_MODE;
> >
> > btw why is it _LONG_MODE and not just _64? :)
>
> I'm pretty sure it got copied from Xen when KVM started adding supporting for
> emulating Xen's hypercalls.  I assume Xen PV actually has a need for identifying
> long mode as opposed to just 64-bit mode, but KVM, not so much.
>
> > > 	vcpu->arch.complete_userspace_io = snp_complete_ext_guest_request;
> > > 	return 0;
> > > }
> >
> > This should work the KVM stored certs nicely but not for the global certs.
> > Although I am not all convinced that global certs is all that valuable but I
> > do not know the history of that, happened before I joined so I let others to
> > comment on that. Thanks,
>
> Aren't the global certs provided by userspace too though?  If all certs are
> ultimately controlled by userspace, I don't see any reason to make the kernel a
> middle-man.

The max blob size is 32KB or so and for 200 VMs it is:
- 6.5MB, all in the userspace so swappable  vs
- 32KB but in the kernel so not swappable.
Sure, a box capable of running 200 VMs must have plenty of RAM but still :)
Plus, GHCB now has to go via the userspace before talking to the PSP 
which was not the case so far (though I cannot think of immediate 
implication right now).


--
Alexey