Sean Christopherson wrote:
> On Wed, Aug 20, 2025, Sagi Shahar wrote:
> > TDX require special handling for VM and VCPU initialization for various
> > reasons:
> > - Special ioctlss for creating VM and VCPU.
> > - TDX registers are inaccessible to KVM.
> > - TDX require special boot code trampoline for loading parameters.
> > - TDX only supports KVM_CAP_SPLIT_IRQCHIP.
>
> Please split this up and elaborate at least a little bit on why each flow needs
> special handling for TDX. Even for someone like me who is fairly familiar with
> TDX, there's too much "Trust me bro" and not enough explanation of why selftests
> really need all of these special paths for TDX.
>
> At least four patches, one for each of your bullet points. Probably 5 or 6, as
> I think the CPUID handling warrants its own patch.
>
> > Hook this special handling into __vm_create() and vm_arch_vcpu_add()
> > using the utility functions added in previous patches.
> >
> > Signed-off-by: Sagi Shahar <sagis@xxxxxxxxxx>
> > ---
> > tools/testing/selftests/kvm/lib/kvm_util.c | 24 ++++++++-
> > .../testing/selftests/kvm/lib/x86/processor.c | 49 ++++++++++++++-----
> > 2 files changed, 61 insertions(+), 12 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> > index b4c8702ba4bd..d9f0ff97770d 100644
> > --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> > +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> > @@ -4,6 +4,7 @@
> > *
> > * Copyright (C) 2018, Google LLC.
> > */
> > +#include "tdx/tdx_util.h"
> > #include "test_util.h"
> > #include "kvm_util.h"
> > #include "processor.h"
> > @@ -465,7 +466,7 @@ void kvm_set_files_rlimit(uint32_t nr_vcpus)
> > static bool is_guest_memfd_required(struct vm_shape shape)
> > {
> > #ifdef __x86_64__
> > - return shape.type == KVM_X86_SNP_VM;
> > + return (shape.type == KVM_X86_SNP_VM || shape.type == KVM_X86_TDX_VM);
> > #else
> > return false;
> > #endif
> > @@ -499,6 +500,12 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus,
> > for (i = 0; i < NR_MEM_REGIONS; i++)
> > vm->memslots[i] = 0;
> >
> > + if (is_tdx_vm(vm)) {
> > + /* Setup additional mem regions for TDX. */
> > + vm_tdx_setup_boot_code_region(vm);
> > + vm_tdx_setup_boot_parameters_region(vm, nr_runnable_vcpus);
> > + }
> > +
> > kvm_vm_elf_load(vm, program_invocation_name);
> >
> > /*
> > @@ -1728,11 +1735,26 @@ void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa)
> > return (void *) ((uintptr_t) region->host_alias + offset);
> > }
> >
> > +static bool is_split_irqchip_required(struct kvm_vm *vm)
> > +{
> > +#ifdef __x86_64__
> > + return is_tdx_vm(vm);
> > +#else
> > + return false;
> > +#endif
> > +}
> > +
> > /* Create an interrupt controller chip for the specified VM. */
> > void vm_create_irqchip(struct kvm_vm *vm)
> > {
> > int r;
> >
> > + if (is_split_irqchip_required(vm)) {
> > + vm_enable_cap(vm, KVM_CAP_SPLIT_IRQCHIP, 24);
> > + vm->has_irqchip = true;
> > + return;
> > + }
>
> Ugh. IMO, this is a KVM bug. Allowing KVM_CREATE_IRQCHIP for a TDX VM is simply
> wrong. It _can't_ work. Waiting until KVM_CREATE_VCPU to fail setup is terrible
> ABI.
>
> If we stretch the meaning of ENOTTY a bit and return that when trying to create
> a fully in-kernel IRQCHIP for a TDX VM, then the selftests code Just Works thanks
> to the code below, which handles the scenario where KVM was be built without
^^^^^^^^^^
I'm not following. Was there supposed to be a patch attached?
Ira
> support for in-kernel I/O APIC (and PIC and PIT).
