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Subject: Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is
 enabled
To:     Borislav Petkov <bp@alien8.de>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        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>, 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>
 <20200821081633.GD12181@zn.tnic>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <3b4ba9e9-dbf6-a094-0684-e68248050758@redhat.com>
Date:   Fri, 21 Aug 2020 11:05:24 +0200
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On 21/08/20 10:16, Borislav Petkov wrote:
> On Fri, Aug 21, 2020 at 10:09:01AM +0200, Paolo Bonzini wrote:
>> One more MSR *is* a big deal: KVM's vmentry+vmexit cost is around 1000
>> cycles, adding 100 clock cycles for 2 WRMSRs is a 10% increase.
> 
> The kernel uses TSC_AUX so we can't reserve it to KVM either.

KVM only uses TSC_AUX while in kernel space, because the kernel hadn't
used it until now.  That's for a good reason:

* if possible, __this_cpu_read(cpu_number) is always faster.

* The kernel can just block preemption at its will and has no need for
the atomic rdtsc+vgetcpu provided by RDTSCP.

So far, the kernel had always used LSL instead of RDPID when
__this_cpu_read was not available.  In one place, RDTSCP is used as an
ordered rdtsc but it discards the TSC_AUX value.  RDPID is also used in
the vDSO but it isn't kernel space.

Hence the assumption that KVM makes (and has made ever since TSC_AUX was
introduced.  What is the difference in speed between LSL and RDPID?  I
don't have a machine that has RDPID to test it, unfortunately.

Paolo