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Subject: Re: [PATCH] KVM/x86: pmu: Fix #GP condition check for RDPMC emulation
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Like Xu <like.xu@linux.intel.com>, kvm@vger.kernel.org,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org
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 <20200708151824.GA22737@linux.intel.com>
 <e285ccb3-29bd-dcb8-73d1-eeee11d72198@redhat.com>
 <20200708154520.GB22737@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <face99cd-f19d-afb3-8f5f-ac5206ba39b4@redhat.com>
Date:   Wed, 8 Jul 2020 17:54:41 +0200
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On 08/07/20 17:45, Sean Christopherson wrote:
> On Wed, Jul 08, 2020 at 05:31:14PM +0200, Paolo Bonzini wrote:
>> The order follows the SDM.  I'm tempted to remove the CR0 check
>> altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's
>> keep it close to what the manual says.
> 
> Heh, it wouldn't surprise me in the least if there's a way to get the SS
> arbyte to hold a non-zero DPL in real mode :-).

I'm not sure if SMM lets you set non-zero SS.DPL in real mode.  It's one
of the few things that are checked with unrestricted guest mode so
there's hope; on the other hand I know for sure that in the past RSM
could get you to VM86 mode with CPL=0, while in VMX it causes vmentry to
fail.

It would be an interesting testcase to write for KVM, to see if you get
a vmentry failure after you set the hidden AR bytes that way and RSM...

Paolo