linux-kernel - Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE

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Message-ID: <e3778bbd-855f-e45a-534e-da73cd423cc6@amd.com>
Date:   Wed, 17 Jan 2018 11:21:18 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, bp@...en8.de,
        dwmw@...zon.co.uk, gregkh@...ux-foundation.org, pjt@...gle.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org, hpa@...or.com,
        tim.c.chen@...ux.intel.com, torvalds@...ux-foundation.org,
        peterz@...radead.org, Dave Hansen <dave.hansen@...el.com>
Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of
 MFENCE_RDTSC

On 1/8/2018 11:01 AM, Paolo Bonzini wrote:
> On 08/01/2018 17:48, Dr. David Alan Gilbert wrote:
>>> If your hypervisor is lying to you about the primary family, then all
>>> bets are off.  I don't expect there will be any production systems doing
>>> this.
>> It's not that an unusual thing to do on qemu/kvm - to specify the lowest
>> common denominator of the set of CPUs in your data centre (for any one
>> vendor); it does tend to get some weird combinations.
> 
> Agreed.  But on a hypervisor we pretty much know that:
> 
> - the MSR_AMD64_DE_CFG doesn't exist unless you have a fix
> 
> - setting the MSR_AMD64_DE_CFG bit to 1 if you have a fix can be done
> independent of the family
> 
> So all KVM needs is a X86_FEATURE_LFENCE_SERIALIZE, it doesn't matter if
> it's because of the family or because Linux has set MSR_F10H_DE_CFG.
> The guest will either try setting the MSR bit and #GP, or it will find
> it already set and do nothing.
> 
> Of course no code for this has been written yet.
> 

Hi Paolo,

What would be the best way to approach the MSR support?  I was thinking of
just recognizing a write to that MSR but not actually doing anything and,
on read, just returning a value with the single bit set if LFENCE is
serializing and not worrying about the full contents of the MSR.  Or I
could save the value so that it could also be host initiated and only
allow the LFENCE serialization bit to be set if the LFENCE_RDTSC feature
is enabled.

Thanks,
Tom

> Paolo
>