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From:   Tony Luck <tony.luck@gmail.com>
Date:   Wed, 9 Jan 2019 10:41:47 -0800
Message-ID: <CA+8MBbLh2JyWg=AF-cftrCfDB8QgRwJ2sKV-S24Jqtu_y1qZrg@mail.gmail.com>
Subject: Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host issue
To:     Borislav Petkov <bp@alien8.de>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Babu Moger <Babu.Moger@amd.com>, X86-ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        =?UTF-8?Q?Jan_H=2E_Sch=C3=B6nherr?= <jschoenh@amazon.de>,
        David Duncan <davdunc@amazon.com>
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On Wed, Jan 9, 2019 at 5:00 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Wed, Jan 09, 2019 at 01:09:31PM +0100, Vitaly Kuznetsov wrote:
> > Hm, why is that? In theory, hypervisors can pass through or emulate the
> > required MSRs...
>
> ...and when the theory becomes reality we'll remove the check.

In practice that may be a long time coming. We don't have many CLOSIDs, or
bits in a cache mask, at the h/w level. If you start trying to
subdivide those resources
to pass a subset to a guest, then you'll quickly find that you have no
flexibility in the
guest to do anything useful.  It would only work if you limited to
two, or perhaps three
guests.

-Tony