linux-kernel - Re: [PATCH v2] x86/speculation/srbds: do not try to turn mitigation off when not supported

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Date:   Thu, 31 Mar 2022 15:18:42 +0200
From:   Ricardo Cañuelo <ricardo.canuelo@...labora.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Mark Gross <mgross@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Johansen <john.johansen@...onical.com>,
        Steve Beattie <sbeattie@...ntu.com>, kernel@...labora.com
Subject: Re: [PATCH v2] x86/speculation/srbds: do not try to turn mitigation off when not supported

Pawan Gupta <pawan.kumar.gupta@...ux.intel.com> writes:

Hi Pawan,

>   void update_srbds_msr(void)
>   {
>   	u64 mcu_ctrl;
>   
> -	if (!boot_cpu_has_bug(X86_BUG_SRBDS))
> -		return;
> -
> -	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> -		return;
> -
> -	if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
> +	if (!boot_cpu_has_bug(X86_BUG_SRBDS) ||
> +	    !boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
>   		return;

Just to clarify, this changes the behavior wrt the hypervisor case:
currently it just bails out of update_srbds_msr(), with your patch it'd
clear RNGDS_MITG_DIS from MSR_IA32_MCU_OPT_CTRL. Is that what you
intended?

Ricardo