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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 19 Aug 2020 19:47:21 -0700
Message-ID: <CALMp9eTDJx9PNDwi+O-_b9O6tBAWEvW-0RAoHYV7inEhw+b42Q@mail.gmail.com>
Subject: Re: [PATCH v4 0/3] Allow user space to restrict and augment MSR emulation
To:     "Graf (AWS), Alexander" <graf@amazon.de>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        "Raslan, KarimAllah" <karahmed@amazon.de>,
        Aaron Lewis <aaronlewis@google.com>,
        kvm list <kvm@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Aug 19, 2020 at 3:09 PM Jim Mattson <jmattson@google.com> wrote:
>
> On Wed, Aug 19, 2020 at 2:46 PM Graf (AWS), Alexander <graf@amazon.de> wrote:
>
> > Special MSRs like EFER also irritate me a bit. We can't really trap on them - most code paths just know they're handled in kernel. Maybe I'll add some sanity checks as well...
>
> Why can't we intercept EFER?

Some MSRs (IA32_GSBASE comes to mind) can't be completely handled by
userspace, even if we do intercept RDMSR and WRMSR. The EFER.LMA bit
also falls into that category, though the rest of the register isn't a
problem (and EFER.LMA is always derivable). Is that what you meant?