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Date:   Thu, 10 Jan 2019 11:53:39 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Tony Luck <tony.luck@gmail.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>,
        Jan =?utf-8?Q?H=2E_Sch=C3=B6nherr?= <jschoenh@amazon.de>,
        David Duncan <davdunc@amazon.com>
Subject: Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host
 issue
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On Thu, Jan 10, 2019 at 11:32:47AM +0100, Vitaly Kuznetsov wrote:
> The other thing is: how can we be sure that there's no hypervisor
> exposing these feature already? Even if open-source hypervisors like
> KVM/Xen don't do it it doesn't prove anything: there are numerous
> proprietary hypervisors and who knows what they do.

Well, we have this thing called CPUID which enumerates features -
regardless of running on baremetal or on a hypervisor. But apparently
some Intel CPUs dropped the ball there. Which caused the f*ckup we are
in right now.

If you really want to not foreclose that feature for guests - and it
sounds like you do - the other thing I can think of is an early quirk
*setting* those feature bits for those Intel CPUs which "forgot" to set
them and then changing the resctrl code to test CPUID bits first.

This way, the kernel is presented with a unified view on what is
supported by the underlying machine - and that machine can be a HV or
baremetal - kernel doesn't care.

> The original issue which triggered the discussion was discovered on
> AWS Xen where the host is buggy and I suggested a simple short-term
> workaround

And I'm sick'n'tired of simple-short term workarounds and band-aids and
the kernel always bending because people dropped the ball and those
being perpetuated, because, sure, we'll add one more "fix" on the pile,
who cares... :-\

If AWS xen is broken, then that should be fixed - not the kernel.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.