linux-kernel - Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

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Date: Tue, 30 Jan 2024 07:44:34 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
 Theodore Ts'o <tytso@....edu>, "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Elena Reshetova <elena.reshetova@...el.com>,
 Jun Nakajima <jun.nakajima@...el.com>, Tom Lendacky
 <thomas.lendacky@....com>, "Kalra, Ashish" <ashish.kalra@....com>,
 Sean Christopherson <seanjc@...gle.com>, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

On 1/30/24 12:30 AM, Kirill A. Shutemov wrote:
> The function rdrand_long() retries 10 times before returning failure to
> the caller. On the other hand, rdseed_long() gives up on the first
> failure.
>
> According to the Intel SDM, both instructions should follow the same
> retry approach. This information can be found in the section titled
> "Random Number Generator Instructions".
>
> To align the behavior of rdseed_long() with rdrand_long(), it should be
> modified to retry 10 times before giving up.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---

Change looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>

Wondering whether this needs to go to stable trees?

>  arch/x86/include/asm/archrandom.h | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/archrandom.h b/arch/x86/include/asm/archrandom.h
> index 02bae8e0758b..918c5880de9e 100644
> --- a/arch/x86/include/asm/archrandom.h
> +++ b/arch/x86/include/asm/archrandom.h
> @@ -33,11 +33,19 @@ static inline bool __must_check rdrand_long(unsigned long *v)
>  
>  static inline bool __must_check rdseed_long(unsigned long *v)
>  {
> +	unsigned int retry = RDRAND_RETRY_LOOPS;
>  	bool ok;
> -	asm volatile("rdseed %[out]"
> -		     CC_SET(c)
> -		     : CC_OUT(c) (ok), [out] "=r" (*v));
> -	return ok;
> +
> +	do {
> +		asm volatile("rdseed %[out]"
> +			     CC_SET(c)
> +			     : CC_OUT(c) (ok), [out] "=r" (*v));
> +
> +		if (ok)
> +			return true;
> +	} while (--retry);
> +
> +	return false;
>  }
>  
>  /*

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer