linux-kernel - Re: [PATCH 08/10] x86/tdx: Unify TDX_HYPERCALL and TDX_MODULE

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Date:   Sat, 15 Jul 2023 13:05:48 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Kai Huang <kai.huang@...el.com>, peterz@...radead.org,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org
Cc:     dave.hansen@...el.com, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org, seanjc@...gle.com,
        pbonzini@...hat.com, kvm@...r.kernel.org, isaku.yamahata@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH 08/10] x86/tdx: Unify TDX_HYPERCALL and TDX_MODULE_CALL
 assembly



<snip>

> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 0f16ba52ae62..a5e77893b2c0 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -51,13 +51,38 @@
>   
>   #define TDREPORT_SUBTYPE_0	0
>   
> +/* Called from __tdx_hypercall() for unrecoverable failure */
> +static noinstr void __tdx_hypercall_failed(void)
> +{
> +	instrumentation_begin();
> +	panic("TDVMCALL failed. TDX module bug?");
> +}

So what's the deal with this instrumentation here. The instruction is 
noinstr, so you want to make just the panic call itself instrumentable?, 
if so where's the instrumentation_end() cal;?No instrumentation_end() 
call. Actually is this complexity really worth it for the failure case?

AFAICS there is a single call site for __tdx_hypercall_failed so why 
noot call panic() directly ?

> +
> +static inline u64 __tdx_hypercall(struct tdx_module_args *args)
> +{
> +	u64 ret;
> +
> +	args->rcx = TDVMCALL_EXPOSE_REGS_MASK;
> +	ret = __tdcall_saved_ret(TDG_VP_VMCALL, args);
> +
> +	/*
> +	 * RAX!=0 indicates a failure of the TDVMCALL mechanism itself and that

nit: Why mention the register explicitly, just say that if 
__tdcall_saved_ret returns non-zero ....

> +	 * something has gone horribly wrong with the TDX module.
> +	 */
> +	if (ret)
> +		__tdx_hypercall_failed();
> +
> +	/* The return status of the hypercall itself is in R10. */
> +	return args->r10;
> +}
> +
>   /*
> - * Wrapper for standard use of __tdx_hypercall with no output aside from
> - * return code.
> + * Wrapper for standard use of __tdx_hypercall() w/o needing any output
> + * register except the return code.
>    */
>   static inline u64 _tdx_hypercall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15)
>   {
> -	struct tdx_hypercall_args args = {
> +	struct tdx_module_args args = {
>   		.r10 = TDX_HYPERCALL_STANDARD,
>   		.r11 = fn,
>   		.r12 = r12,

<snip>