linux-kernel - Re: [PATCH v13 05/22] x86/virt/tdx: Handle SEAMCALL no entropy error in common code

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Message-ID: <0676101a-e781-81e0-2e0f-7f5e72595e5c@intel.com>
Date:   Fri, 8 Sep 2023 09:21:19 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     x86@...nel.org, kirill.shutemov@...ux.intel.com,
        tony.luck@...el.com, peterz@...radead.org, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, hpa@...or.com, seanjc@...gle.com,
        pbonzini@...hat.com, david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ashok.raj@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, ying.huang@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, nik.borisov@...e.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v13 05/22] x86/virt/tdx: Handle SEAMCALL no entropy error
 in common code

On 8/25/23 05:14, Kai Huang wrote:
> Some SEAMCALLs use the RDRAND hardware and can fail for the same reasons
> as RDRAND.  Use the kernel RDRAND retry logic for them.
> 
> There are three __seamcall*() variants.  Add a macro to do the SEAMCALL
> retry in the common code and define a wrapper for each __seamcall*()
> variant.
> 
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
> 
> v12 -> v13:
>  - New implementation due to TDCALL assembly series.
> 
> ---
>  arch/x86/include/asm/tdx.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index a252328734c7..cfae8b31a2e9 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -24,6 +24,11 @@
>  #define TDX_SEAMCALL_GP			(TDX_SW_ERROR | X86_TRAP_GP)
>  #define TDX_SEAMCALL_UD			(TDX_SW_ERROR | X86_TRAP_UD)
>  
> +/*
> + * TDX module SEAMCALL leaf function error codes
> + */
> +#define TDX_RND_NO_ENTROPY	0x8000020300000000ULL
> +
>  #ifndef __ASSEMBLY__
>  
>  /*
> @@ -82,6 +87,28 @@ u64 __seamcall(u64 fn, struct tdx_module_args *args);
>  u64 __seamcall_ret(u64 fn, struct tdx_module_args *args);
>  u64 __seamcall_saved_ret(u64 fn, struct tdx_module_args *args);
>  
> +#include <asm/archrandom.h>
> +
> +#define SEAMCALL_NO_ENTROPY_RETRY(__seamcall_func, __fn, __args)	\
> +({									\
> +	int ___retry = RDRAND_RETRY_LOOPS;				\
> +	u64 ___sret;							\
> +									\
> +	do {								\
> +		___sret = __seamcall_func((__fn), (__args));		\
> +	} while (___sret == TDX_RND_NO_ENTROPY && --___retry);		\
> +	___sret;							\
> +})

This is a *LOT* less eye-bleedy if you do it without macros:


typedef u64 (*sc_func_t)(u64 fn, struct tdx_module_args *args);

static inline
u64 sc_retry(sc_func_t func, u64 fn, struct tdx_module_args *args)
{
        int retry = RDRAND_RETRY_LOOPS;
        u64 ret;

        do {
                ret = func(fn, args);
        } while (ret == TDX_RND_NO_ENTROPY && --retry);

        return ret;
}

#define seamcall(_fn, _args)           sc_retry(_seamcall,
(_fn), (_args))
#define seamcall_ret(_fn, _args)       sc_retry(_seamcall_ret,
(_fn), (_args))
#define seamcall_saved_ret(_fn, _args) sc_retry(_seamcall_saved_ret,
(_fn), (_args))

The compiler can figure it out and avoid making func() an indirect call
since it knows the call location at compile time.

You can also do the seamcall() #define as a static inline, but it does
take up more screen real estate.  Oh, and going a wee bit over 80
columns is OK for those #defines.