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

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Date:   Fri, 21 Aug 2020 11:37:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 11:28, 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.