linux-kernel - Re: [RFC PATCH v5 015/104] KVM: TDX: add a helper function for KVM to issue SEAMCALL

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Date:   Sun, 13 Mar 2022 15:11:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 015/104] KVM: TDX: add a helper function for KVM to
 issue SEAMCALL

On 3/4/22 20:48, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> TODO: Consolidate seamcall helper function with TDX host/guest patch series.
> For now, this is kept to make this patch series compile/work.
> 
> A VMM interacts with the TDX module using a new instruction (SEAMCALL).  A
> TDX VMM uses SEAMCALLs where a VMX VMM would have directly interacted with
> VMX instructions.  For instance, a TDX VMM does not have full access to the
> VM control structure corresponding to VMX VMCS.  Instead, a VMM induces the
> TDX module to act on behalf via SEAMCALLs.
> 
> Add a helper function for KVM C code to execute SEAMCALL instruction to
> hide its SEAMCALL ABI details.  Although the x86 TDX host patch series
> defines a similar wrapper, the KVM TDX patch series defines its own because
> KVM TDX case is performance-critical, unlike the x86 TDX one that does
> one-time initialization.  The difference is that the KVM TDX one is defined
> as a static inline function without an error check that is known to not
> happen so that compiler can optimize it better.  The wrapper fiction in the
> x86 TDX host patch is defined as a function written in assembly code with
> error check so that it can detect errors that can occur only during the
> initialization.

I assume whatever survives of this patch will be merged in the previous one.

Paolo

> Co-developed-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>   arch/x86/kvm/vmx/seamcall.h | 23 +++++++++++++++++++++++
>   1 file changed, 23 insertions(+)
>   create mode 100644 arch/x86/kvm/vmx/seamcall.h
> 
> diff --git a/arch/x86/kvm/vmx/seamcall.h b/arch/x86/kvm/vmx/seamcall.h
> new file mode 100644
> index 000000000000..604792e9a59f
> --- /dev/null
> +++ b/arch/x86/kvm/vmx/seamcall.h
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __KVM_VMX_SEAMCALL_H
> +#define __KVM_VMX_SEAMCALL_H
> +
> +#ifdef CONFIG_INTEL_TDX_HOST
> +
> +#ifdef __ASSEMBLY__
> +
> +.macro seamcall
> +	.byte 0x66, 0x0f, 0x01, 0xcf
> +.endm
> +
> +#else
> +
> +struct tdx_module_output;
> +u64 kvm_seamcall(u64 op, u64 rcx, u64 rdx, u64 r8, u64 r9, u64 r10,
> +		struct tdx_module_output *out);
> +
> +#endif /* !__ASSEMBLY__ */
> +
> +#endif	/* CONFIG_INTEL_TDX_HOST */
> +
> +#endif /* __KVM_VMX_SEAMCALL_H */