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 09:25:28 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ricardo Cañuelo <ricardo.canuelo@...labora.com>,
        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

On Thu, Mar 31, 2022 at 05:25:27PM +0200, Borislav Petkov wrote:
>On Thu, Mar 31, 2022 at 03:18:42PM +0200, Ricardo Cañuelo wrote:
>> 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?

Yes, this should be fine as update_srbds_msr() would also check for
X86_FEATURE_SRBDS_CTRL. If a hypervisor doesn't want a guest to write to
the MSR it should not export X86_FEATURE_SRBDS_CTRL.

>Just do the simple thing I pasted earlier - no need to rewrite the whole
>function for no good reason.

I was trying to address earlier comment on split logic in two functions.
I am okay with keeping it as is (and just adding X86_FEATURE_SRBDS_CTRL
check in update_srbds_msr()).

Thanks,
Pawan