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

From: "Ricardo Cañuelo" <ricardo.canuelo@collabora.com>
To: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>,
	linux-kernel@vger.kernel.org,
	Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
	Mark Gross <mgross@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	John Johansen <john.johansen@canonical.com>,
	Steve Beattie <sbeattie@ubuntu.com>,
	kernel@collabora.com
Subject: Re: [PATCH v2] x86/speculation/srbds: do not try to turn mitigation off when not supported
Date: Thu, 31 Mar 2022 15:18:42 +0200	[thread overview]
Message-ID: <87lewqz2dp.fsf@rcn-XPS-13-9305.i-did-not-set--mail-host-address--so-tickle-me> (raw)
In-Reply-To: <20220331084536.y4sl7qcfzltsnnew@guptapa-desk>

Pawan Gupta <pawan.kumar.gupta@linux.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