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

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Message-ID: <20200615082858.GC14668@zn.tnic>
Date:   Mon, 15 Jun 2020 10:28:58 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Cc:     linux-kernel@...r.kernel.org, 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>
Subject: Re: [PATCH] x86/speculation/srbds: do not try to turn mitigation off
 when not supported

On Tue, Jun 09, 2020 at 02:43:13PM -0300, Thadeu Lima de Souza Cascardo wrote:
> When SRBDS is mitigated by TSX OFF, update_srbds_msr will still read and

Are you talking about this case in srbds_select_mitigation():

        if ((ia32_cap & ARCH_CAP_MDS_NO) && !boot_cpu_has(X86_FEATURE_RTM))
                srbds_mitigation = SRBDS_MITIGATION_TSX_OFF;

?

and you have a system which:

         * Check to see if this is one of the MDS_NO systems supporting
         * TSX that are only exposed to SRBDS when TSX is enabled.

i.e., no SRBDS microcode for it and the fix is to disable TSX?

If so, I think the right fix should be to do:

	if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
		return;

in update_srbds_msr() with a comment above it explaining why that check
is being done.

Hmmm.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette