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

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Message-ID: <6904c9e7a7090_23147410037@iweiny-mobl.notmuch>
Date: Fri, 31 Oct 2025 09:38:31 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Sagi Shahar <sagis@...gle.com>, <linux-kselftest@...r.kernel.org>, "Paolo
 Bonzini" <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, "Sean
 Christopherson" <seanjc@...gle.com>, 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>,
	Ira Weiny <ira.weiny@...el.com>, Chao Gao <chao.gao@...el.com>, Chenyi Qiang
	<chenyi.qiang@...el.com>
CC: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH v12 22/23] KVM: selftests: Add ucall support for TDX

Sagi Shahar wrote:
> From: Ackerley Tng <ackerleytng@...gle.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]