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Subject: Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is
 enabled
To:     Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, 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>, kvm@vger.kernel.org,
        Tom Lendacky <thomas.lendacky@amd.com>
References: <20200821025050.32573-1-sean.j.christopherson@intel.com>
 <20200821074743.GB12181@zn.tnic>
 <3eb94913-662d-5423-21b1-eaf75635142a@redhat.com>
 <87r1s0gxfj.fsf@nanos.tec.linutronix.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <f8597183-5a14-de7c-330b-46b9bf015d0c@redhat.com>
Date:   Fri, 21 Aug 2020 11:37:13 +0200
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On 21/08/20 11:28, Thomas Gleixner wrote:
> We all know that MSRs are slow, but as a general rule I have to make it
> entirely clear that the kernel has precedence over KVM.

I totally agree.  I just don't think that it matters _in this case_,
because the kernel hardly has any reason to use TSC_AUX while in ring0.

Paolo

> If the kernel wants to use an MSR for it's own purposes then KVM has to
> deal with that and not the other way round. Preventing the kernel from
> using a facility freely is not an option ever.
> 
> The insanities of KVM performance optimizations have bitten us more than
> once.
> 
> For this particular case at hand I don't care much and we should just
> rip the whole RDPID thing out unconditionally. We still have zero
> numbers about the performance difference vs. LSL.