linux-kernel - Re: [PATCH v12 20/23] KVM: selftests: Add support for TDX TDCALL from guest

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Message-ID: <6904c3834e3c0_231474100ca@iweiny-mobl.notmuch>
Date: Fri, 31 Oct 2025 09:11:15 -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 20/23] KVM: selftests: Add support for TDX TDCALL
 from guest

Sagi Shahar wrote:
> From: Erdem Aktas <erdemaktas@...gle.com>
> 
> Add support for TDX guests to issue TDCALLs to the TDX module.

Generally it is nice to have more details.  As someone new to TDX I
have to remind myself what a TDCALL is.  And any random kernel developer
reading this in the future will likely have even less clue than me.

Paraphrased from the spec:

TDCALL is the instruction used by the guest TD software (in TDX non-root
mode) to invoke guest-side TDX functions.  TDG.VP.VMCALL helps invoke
services from the host VMM.

Add support for TDX guests to invoke services from the host VMM.

> 
> Signed-off-by: Erdem Aktas <erdemaktas@...gle.com>
> Co-developed-by: Sagi Shahar <sagis@...gle.com>
> Signed-off-by: Sagi Shahar <sagis@...gle.com>
> ---

[snip]

> diff --git a/tools/testing/selftests/kvm/include/x86/tdx/tdcall.h b/tools/testing/selftests/kvm/include/x86/tdx/tdcall.h
> new file mode 100644
> index 000000000000..60c70646f876
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/include/x86/tdx/tdcall.h

[snip]

> +
> +/*
> + * Used in __tdx_tdcall() to pass down and get back registers' values of
> + * the TDCALL instruction when requesting services from the VMM.
> + *
> + * This is a software only structure and not part of the TDX module/VMM ABI.

This is a good comment.

> + */
> +struct tdx_tdcall_args {
> +	u64 r10;
> +	u64 r11;
> +	u64 r12;
> +	u64 r13;
> +	u64 r14;
> +	u64 r15;
> +};
> +

[snip]

> +
> +/*
> + * Bitmasks of exposed registers (with VMM).
> + */
> +#define TDX_R10		BIT(10)
> +#define TDX_R11		BIT(11)
> +#define TDX_R12		BIT(12)
> +#define TDX_R13		BIT(13)
> +#define TDX_R14		BIT(14)
> +#define TDX_R15		BIT(15)
> +
> +/*
> + * These registers are clobbered to hold arguments for each
> + * TDVMCALL. They are safe to expose to the VMM.

I'm not sure what this comment means by being 'safe to expose to the VMM'?
They are all overwritten per the data specified correct?

> + * Each bit in this mask represents a register ID. Bit field
> + * details can be found in TDX GHCI specification, section
> + * titled "TDCALL [TDG.VP.VMCALL] leaf".

TDX GHCI specification v1.5, March 2023
2.4.1 TDCALL [TDG.VP.VMCALL] leaf

This nails down any issues which may arise if the module/spec changes.

Ira

[snip]