linux-kernel - Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is enabled

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Message-ID: <1442e559-dde4-70f6-85ac-58109cf81c16@redhat.com>
Date:   Fri, 21 Aug 2020 11:44:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Chang Seok Bae <chang.seok.bae@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sasha Levin <sashal@...nel.org>, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is
 enabled

On 21/08/20 11:22, Borislav Petkov wrote:
>> Hence the assumption that KVM makes (and has made ever since TSC_AUX was
>> introduced.
> And *this* is the problem. KVM can't just be grabbing MSRs and claiming
> them because it started using them first. You guys need to stop this. If
> it is a shared resource, there better be an agreement about sharing it.

It's not like we grab MSRs every day.  The user-return notifier restores
6 MSRs (7 on very old processors).  The last two that were added were
MSR_TSC_AUX itself in 2009 (!) and MSR_IA32_TSX_CTRL last year.

Paolo