Re: [PATCH v12 22/23] KVM: selftests: Add ucall support for TDX

Ira Weiny
2025-10-31T14:36:14Z

Sagi Shahar wrote:
> From: Ackerley Tng <ackerleytng@google.com>
>
> ucalls for non-Coco VMs work by having the guest write to the rdi
> register, then perform an io instruction to exit to the host. The host
> then reads rdi using kvm_get_regs().
>
> CPU registers can't be read using kvm_get_regs() for TDX, so TDX
> guests use MMIO to pass the struct ucall's hva to the host. MMIO was
> chosen because it is one of the simplest (hence unlikely to fail)
> mechanisms that support passing 8 bytes from guest to host.
>

[snip]

> index 1265cecc7dd1..fae6f37b0bcd 100644
> --- a/tools/testing/selftests/kvm/lib/x86/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/x86/ucall.c
> @@ -5,11 +5,35 @@
> * Copyright (C) 2018, Red Hat, Inc.
> */
> #include "kvm_util.h"
> +#include "tdx/tdx.h"
> +#include "tdx/tdx_util.h"
>
> #define UCALL_PIO_PORT ((uint16_t)0x1000)
>
> +static uint8_t vm_type;
> +static vm_paddr_t host_ucall_mmio_gpa;
> +static vm_paddr_t ucall_mmio_gpa;
> +
> +void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)
> +{
> + vm_type = vm->type;
> + sync_global_to_guest(vm, vm_type);
> +
> + if (is_tdx_vm(vm)) {
> + host_ucall_mmio_gpa = ucall_mmio_gpa = mmio_gpa;
> + ucall_mmio_gpa |= vm->arch.s_bit;
> + }
> +
> + sync_global_to_guest(vm, ucall_mmio_gpa);

Is this needed for non-tdx VMs?

Ira

[snip]