linux-kernel - Re: [PATCH v9 15/19] KVM: selftests: Hook TDX support to vm and vcpu creation

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Message-ID: <CAAhR5DHPMPOb2XCJodyNMf2RTQfTZpAaCGMg6WeWxSWPLtkO4Q@mail.gmail.com>
Date: Tue, 26 Aug 2025 15:29:28 -0500
From: Sagi Shahar <sagis@...gle.com>
To: Ira Weiny <ira.weiny@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>, linux-kselftest@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Ackerley Tng <ackerleytng@...gle.com>, Ryan Afranji <afranji@...gle.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Erdem Aktas <erdemaktas@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Roger Wang <runanwang@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, "Pratik R. Sampat" <pratikrajesh.sampat@....com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Chenyi Qiang <chenyi.qiang@...el.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v9 15/19] KVM: selftests: Hook TDX support to vm and vcpu creation

On Tue, Aug 26, 2025 at 3:14 PM Ira Weiny <ira.weiny@...el.com> wrote:
>
> 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@...gle.com>
> > > ---
> > >  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?
>

I think Sean refers to the original implementation which was out of
the scope for the git diff so it was left out of the patch:

/*
 * Allocate a fully in-kernel IRQ chip by default, but fall back to a
 * split model (x86 only) if that fails (KVM x86 allows compiling out
 * support for KVM_CREATE_IRQCHIP).
 */
r = __vm_ioctl(vm, KVM_CREATE_IRQCHIP, NULL);
if (r && errno == ENOTTY && kvm_has_cap(KVM_CAP_SPLIT_IRQCHIP))
        vm_enable_cap(vm, KVM_CAP_SPLIT_IRQCHIP, 24);
else
        TEST_ASSERT_VM_VCPU_IOCTL(!r, KVM_CREATE_IRQCHIP, r, vm);

/*
 * Allocate a fully in-kernel IRQ chip by default, but fall back to a
 * split model (x86 only) if that fails (KVM x86 allows compiling out
 * support for KVM_CREATE_IRQCHIP).
 */
r = __vm_ioctl(vm, KVM_CREATE_IRQCHIP, NULL);
if (r && errno == ENOTTY && kvm_has_cap(KVM_CAP_SPLIT_IRQCHIP))
vm_enable_cap(vm, KVM_CAP_SPLIT_IRQCHIP, 24);
else
TEST_ASSERT_VM_VCPU_IOCTL(!r, KVM_CREATE_IRQCHIP, r, vm);
/*
* Allocate a fully in-kernel IRQ chip by default, but fall back to a
* split model (x86 only) if that fails (KVM x86 allows compiling out
* support for KVM_CREATE_IRQCHIP).
*/
r = __vm_ioctl(vm, KVM_CREATE_IRQCHIP, NULL);
if (r && errno == ENOTTY && kvm_has_cap(KVM_CAP_SPLIT_IRQCHIP))
vm_enable_cap(vm, KVM_CAP_SPLIT_IRQCHIP, 24);
else
TEST_ASSERT_VM_VCPU_IOCTL(!r, KVM_CREATE_IRQCHIP, r, vm);

> Ira
>
> > support for in-kernel I/O APIC (and PIC and PIT).
>
>