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

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Message-ID: <20200821092237.GF12181@zn.tnic>
Date:   Fri, 21 Aug 2020 11:22:37 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
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 Fri, Aug 21, 2020 at 11:05:24AM +0200, Paolo Bonzini wrote:
> ... RDTSCP is used as an ordered rdtsc but it discards the TSC_AUX
> value.

... and now because KVM is using it, the kernel can forget using
TSC_AUX. Are you kidding me?!

> 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.

-- 
Regards/Gruss,
    Boris.

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