Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 21 Aug 2020 09:47:43 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@intel.com>,
        Chang Seok Bae <chang.seok.bae@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sasha Levin <sashal@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is
 enabled
Message-ID: <20200821074743.GB12181@zn.tnic>
References: <20200821025050.32573-1-sean.j.christopherson@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200821025050.32573-1-sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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