On 10/2/2026 3:37 AM, Lisa Wang wrote: > From: Sean Christopherson <[email protected]> > > Add support for doing ucalls on TDX by abusing TDVMCALL_REPORT_FATAL_ERROR > to pass the address of the payload to the host. The "fatal error" TDVMCALL > is perfectly suited for passing information to host userspace, is both the ^ as
> TDX Module and KVM allow the guest to pass (almost) all registers to the > host, i.e. provide enough of a data payload to make a collision with a real > fatal error practically impossible. > > TDX can't use port I/O, as the TDX ABI doesn't allow the guest to share > arbitrary register state with the host on a port I/O exit, and the port I/O > data payload is limited to 4 bytes, i.e. would potentially truncate the > ucall address. > > Alternatively, TDX could use MMIO, but using a magic emulated MMIO address > is fragile (see the TODO in __vm_create()), especially for TDX since TDX > doesn't support read-only memslots, i.e. doesn't have line of sight towards > addressing the TODO. E.g. TDX could hardcode the address to something that > is all but guaranteed to be unused on x86, e.g. the I/O APIC base address > or the HPET address, but that doesn't truly address the fragility concerns, > and it's ugly because ucall_arch_init() would completely ignore the passed > in @mmio_gpa despite obviously utilizing emulated MMIO. > > Signed-off-by: Sean Christopherson <[email protected]> > Signed-off-by: Lisa Wang <[email protected]> Also one nit below. otherwise, Reviewed-by: Binbin Wu <[email protected]> > --- > tools/testing/selftests/kvm/include/x86/tdx/tdx.h | 7 ++++++ > tools/testing/selftests/kvm/lib/x86/ucall.c | 27 > ++++++++++++++++++++++- > 2 files changed, 33 insertions(+), 1 deletion(-) > > diff --git a/tools/testing/selftests/kvm/include/x86/tdx/tdx.h > b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h > index d41a1efc8a63..9982aaaba885 100644 > --- a/tools/testing/selftests/kvm/include/x86/tdx/tdx.h > +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h > @@ -4,6 +4,13 @@ > > #include <linux/types.h> > > +/* TDX hypercall Leaf IDs */ > +#define TDVMCALL_GET_TD_VM_CALL_INFO 0x10000 > +#define TDVMCALL_MAP_GPA 0x10001 > +#define TDVMCALL_GET_QUOTE 0x10002 > +#define TDVMCALL_REPORT_FATAL_ERROR 0x10003 > +#define TDVMCALL_SETUP_EVENT_NOTIFY_INTERRUPT 0x10004 > + > u64 __tdvmcall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15); > > #endif /* SELFTEST_KVM_TDX_TDX_H */ > diff --git a/tools/testing/selftests/kvm/lib/x86/ucall.c > b/tools/testing/selftests/kvm/lib/x86/ucall.c > index c003df3c7b8a..c1cd32d68464 100644 > --- a/tools/testing/selftests/kvm/lib/x86/ucall.c > +++ b/tools/testing/selftests/kvm/lib/x86/ucall.c > @@ -5,8 +5,28 @@ > * Copyright (C) 2018, Red Hat, Inc. > */ > #include "kvm_util.h" > +#include "tdx/tdx.h" > +#include "tdx/tdx_util.h" > > -#define UCALL_PIO_PORT ((u16)0x1000) > +#define UCALL_PIO_PORT ((u16)0x1000) > +#define UCALL_TDX_MAGIC 0xabacadabaULL > + > +static void ucall_tdx_do_ucall(gva_t uc) > +{ > + __tdvmcall(TDVMCALL_REPORT_FATAL_ERROR, UCALL_TDX_MAGIC, uc, 0, 0); > +} > + > +static void *ucall_tdx_get_ucall(struct kvm_vcpu *vcpu) > +{ > + struct kvm_run *run = vcpu->run; > + > + if (run->exit_reason == KVM_EXIT_SYSTEM_EVENT && > + run->system_event.type == KVM_SYSTEM_EVENT_TDX_FATAL && > + run->system_event.data[12] == UCALL_TDX_MAGIC) > + return (void *)(run->system_event.data[13]); Nit: It's more readable to have comment for data[12] and data[13] to indicate they are the R12 and R13 passed. > + > + return NULL; > +} > > static void ucall_x86_do_ucall(gva_t uc) > { > @@ -37,6 +57,11 @@ static struct { > > void ucall_arch_init(struct kvm_vm *vm, gpa_t mmio_gpa) > { > + if (is_tdx_vm(vm)) { > + ucall_x86_ops.do_ucall = ucall_tdx_do_ucall; > + ucall_x86_ops.get_ucall = ucall_tdx_get_ucall; > + } > + > sync_global_to_guest(vm, ucall_x86_ops); > } > >
