linux-kernel - Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting

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Date:   Fri, 27 Jul 2018 10:15:32 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Kyle Huey <me@...ehuey.com>
Cc:     "Robert O'Callahan" <robert@...llahan.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Len Brown <len.brown@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        David Matlack <dmatlack@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andi Kleen <andi@...stfloor.org>,
        LKML <linux-kernel@...r.kernel.org>,
        user-mode-linux-devel@...ts.sourceforge.net,
        user-mode-linux-user@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting

On Wed, Feb 8, 2017 at 12:09 AM, Kyle Huey <me@...ehuey.com> wrote:
> Hardware support for faulting on the cpuid instruction is not required to
> emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant
> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
> cpuid-induced VM exit checks the cpuid faulting state and the CPL.
> kvm_require_cpl is even kind enough to inject the GP fault for us.
>
> Signed-off-by: Kyle Huey <khuey@...ehuey.com>
> Reviewed-by: David Matlack <dmatlack@...gle.com>

I have a couple of concerns about portions of this patch:

1) There are some backward compatibility issues:
  A) Suppose we have an old userspace that doesn't know it needs to
zero MSR_PLATFORM_INFO to preserve existing behavior (to the extent
possible). If a VM starts on a new kernel it could set the bit in
MSR_MISC_FEATURES_ENABLES that enables CPUID faulting. On
live-migration to an old kernel, that bit would be lost.
  B) With either an old userspace or a new userspace, as a VM migrates
between old and new kernels, the behavior of RDMSR with ECX set to
either MSR_PLATFORM_INFO or MSR_MISC_FEATURES_ENABLES will vary
depending on which kernel the VM is currently running on.
  Ideally, I think this new functionality should be guarded by a KVM
capability that has to be enabled from userspace.

2) This doesn't really play well with volume 3 of the SDM, section
18.7.3, where Intel instructs developers to use
MSR_PLATFORM_INFO[15:8] to determine the TSC frequency for a variety
of microarchitectures. When reads of this MSR raised #GP, it was
pretty clear that one couldn't get the TSC frequency that way, but I
don't think many consumers would specifically check for a 0 in that
field when the RDMSR succeeds. If a guest hypervisor used that value
in the computation of the TSC scaling factor for a VMCS12, for
example, it might be surprised to get a #DE.