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

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Message-ID: <20200821074743.GB12181@zn.tnic>
Date:   Fri, 21 Aug 2020 09:47:43 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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>,
        Paolo Bonzini <pbonzini@...hat.com>, 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 Thu, Aug 20, 2020 at 07:50:50PM -0700, Sean Christopherson wrote:
> +	 * Disallow RDPID if KVM is enabled as it may consume a guest's TSC_AUX
> +	 * if an NMI arrives in KVM's run loop.  KVM loads guest's TSC_AUX on
> +	 * VM-Enter and may not restore the host's value until the CPU returns
> +	 * to userspace, i.e. KVM depends on the kernel not using TSC_AUX.
>  	 */

And frankly, this is really unfair. The kernel should be able to use any
MSR. IOW, KVM needs to be fixed here. I'm sure it context-switches other
MSRs so one more MSR is not a big deal.

-- 
Regards/Gruss,
    Boris.

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