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From:   Christophe de Dinechin <christophe.de.dinechin@gmail.com>
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Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure
In-Reply-To: <20180130204623.583b1a7a@alans-desktop>
Date:   Wed, 31 Jan 2018 11:05:15 +0100
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Eduardo Habkost <ehabkost@redhat.com>,
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        Andrea Arcangeli <aarcange@redhat.com>,
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        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
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        Jun Nakajima <jun.nakajima@intel.com>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
        KVM list <kvm@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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To:     Alan Cox <gnomes@lxorguk.ukuu.org.uk>
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> On 30 Jan 2018, at 21:46, Alan Cox <gnomes@lxorguk.ukuu.org.uk> wrote:
>=20
>> If you are ever going to migrate to Skylake, I think you should just
>> always tell the guests that you're running on Skylake. That way the
>> guests will always assume the worst case situation wrt Specte.
>=20
> Unfortunately if you do that then guest may also decide to use other
> Skylake hardware features and pop its clogs when it finds out its =
actually
> running on Westmere or SandyBridge.
>=20
> So you need to be able to both lie to the OS and user space via cpuid =
and
> also have a second 'but do skylake protections' that only mitigation
> aware software knows about.

Yes. The most desirable lie is different depending on whether you want =
to
allow virtualization features such as migration (where you=E2=80=99d =
gravitate
towards a CPU with less features) or whether you want to allow =
mitigation
(where you=E2=80=99d rather present the most fragile CPUID, probably =
Skylake).

Looking at some recent patches, I=E2=80=99m concerned that the code =
being added
often assumes that the CPUID is the correct way to get that info.
I do not think this is correct. You really want specific information =
about
the host CPUID, not whatever KVM CPUID emulation makes up.