linux-kernel - Re: [V4 6/8] KVM: selftests: add library for creating/interacting with SEV guests

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Date:   Wed, 19 Oct 2022 16:34:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        marcorr@...gle.com, michael.roth@....com, thomas.lendacky@....com,
        joro@...tes.org, mizhang@...gle.com, pbonzini@...hat.com,
        andrew.jones@...ux.dev
Subject: Re: [V4 6/8] KVM: selftests: add library for creating/interacting
 with SEV guests

On Tue, Oct 18, 2022, Peter Gonda wrote:
> On Mon, Oct 17, 2022 at 2:34 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Mon, Oct 17, 2022, Peter Gonda wrote:
> > > I think this means we don't need to add VM_MODE_PXXV48_4K_SEV since we
> > > can set up the c-bit from inside of vm_sev_create_*(), thoughts?
> >
> > Configuring the C-bit inside vm_sev_create_*() won't work (at least not well).
> > The C-bit needs to be known before kvm_vm_elf_load(), i.e. can't be handled after
> > __vm_create(), and needs to be tracked inside the VM, i.e. can't be handled before
> > __vm_create().
> >
> > The proposed kvm_init_vm_address_properties() seems like the best fit since the
> > C-bit (and TDX's S-bit) is stolen from GPA space, i.e. directly affects the other
> > values computed in that path.
> >
> > As for the kvm_vm_arch allocation ugliness, when we talked off-list I didn't
> > consider the need to allocate in kvm_init_vm_address_properties().  That's quite
> > gross, especially since the pointer will be larger than the thing being allocated.
> >
> > With that in mind, adding .../include/<arch>/kvm_util.h so that "struct kvm_vm_arch"
> > can be defined and referenced directly doesn't seem so bad.  Having to stub in the
> > struct for the other architectures is annoying, but not the end of the world.
> 
> I'll make "struct kvm_vm_arch" a non pointer member, so adding
> /include/<arch>/kvm_util.h files.
> 
> But I think we do not need VM_MODE_PXXV48_4K_SEV, see:

I really don't want to open code __vm_create() with a slight tweak.  E.g. the
below code will be broken by Ricardo's series to add memslot0 is moved out of
____vm_create()[1], and kinda sorta be broken again by Vishal's series to add an
arch hook to __vm_create()[2].

AFAICT, there is no requirement that KVM_SEV_INIT be called before computing the
C-Bit, the only requirement is that KVM_SEV_INIT is called before adding vCPUs.

[1] https://lore.kernel.org/all/20221017195834.2295901-8-ricarkol@google.com
[2] https://lore.kernel.org/all/YzsC4ibDqGh5qaP9@google.com

> struct kvm_vm *vm_sev_create_with_one_vcpu(uint32_t policy, void *guest_code,
>                                            struct kvm_vcpu **cpu)
> {
>         enum vm_guest_mode mode = VM_MODE_PXXV48_4K;
>         uint64_t nr_pages = vm_nr_pages_required(mode, 1, 0);
>         struct kvm_vm *vm;
>         uint8_t measurement[512];
>         int i;
> 
>         vm = ____vm_create(mode, nr_pages);
> 
>         kvm_sev_ioctl(vm, KVM_SEV_INIT, NULL);
> 
>         configure_sev_pte_masks(vm);
> 
>         *cpu = vm_vcpu_add(vm, 0, guest_code);
>         kvm_vm_elf_load(vm, program_invocation_name);
> 
>         sev_vm_launch(vm, policy);
> 
>         /* Dump the initial measurement. A test to actually verify it
> would be nice. */
>         sev_vm_launch_measure(vm, measurement);
>         pr_info("guest measurement: ");
>         for (i = 0; i < 32; ++i)
>                 pr_info("%02x", measurement[i]);
>         pr_info("\n");
> 
>         sev_vm_launch_finish(vm);
> 
>         return vm;
> }