linux-kernel - Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL

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Date:   Fri, 1 Oct 2021 15:07:34 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Jane Malalane <jane.malalane@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>
CC:     <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Pu Wen <puwen@...on.cn>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yazen Ghannam <Yazen.Ghannam@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Huang Rui <ray.huang@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Kim Phillips <kim.phillips@....com>, <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

On 01/10/2021 14:33, Jane Malalane wrote:
> Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which
> makes it unsafe to migrate in a virtualised environment as the
> properties across the migration pool might differ.
>
> Zen3 adds the NullSelectorClearsBase bit to indicate that loading
> a NULL segment selector zeroes the base and limit fields, as well as
> just attributes. Zen2 also has this behaviour but doesn't have the
> NSCB bit.
>
> When virtualised, NSCB might be cleared for migration safety,
> therefore we must not probe. Always honour the NSCB bit in this case,
> as the hypervisor is expected to synthesize it in the Zen2 case.
>
> Signed-off-by: Jane Malalane <jane.malalane@...rix.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>