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

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Message-ID: <4d9a1f0d-a401-3f0f-9ee2-dd42f4b4716a@amd.com>
Date:   Mon, 8 Jan 2018 08:47:15 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     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@...el.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of
 MFENCE_RDTSC

On 1/8/2018 5:10 AM, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Andrew Cooper wrote:
> 
>> On 08/01/18 10:08, Thomas Gleixner wrote:
>>> On Sat, 6 Jan 2018, tip-bot for Tom Lendacky wrote:
>>>
>>>> Commit-ID:  0bf17c102177d5da9363bf8b1e4704b9996d5079
>>>> Gitweb:     https://git.kernel.org/tip/0bf17c102177d5da9363bf8b1e4704b9996d5079
>>>> Author:     Tom Lendacky <thomas.lendacky@....com>
>>>> AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600
>>>> Committer:  Thomas Gleixner <tglx@...utronix.de>
>>>> CommitDate: Sat, 6 Jan 2018 21:57:40 +0100
>>>>
>>>> x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC
>>>>
>>>> With LFENCE now a serializing instruction, set the LFENCE_RDTSC
>>>> feature since the LFENCE instruction has less overhead than the
>>>> MFENCE instruction.
>>> Second thoughts on that. As pointed out by someone in one of the insane
>>> long threads:
>>>
>>> What happens if the kernel runs as a guest and
>>>
>>>   - the hypervisor did not set the LFENCE to serializing on the host
>>>
>>>   - the hypervisor does not allow writing MSR_AMD64_DE_CFG
>>>
>>> That would bring the guest into a pretty bad state or am I missing
>>> something essential here?
>>
>> What I did in Xen was to attempt to set it, then read it back and see. 
>> If LFENCE still isn't serialising, using repoline is the only available
>> mitigation.
>>
>> My understanding from the folk at AMD is that retpoline is safe to use,
>> but has higher overhead than the LFENCE approach.

Correct, the retpoline will work, it just takes more cycles.

> 
> That still does not help vs. rdtsc_ordered() and LFENCE_RDTSC ...

Ok, I can add the read-back check before setting the feature flag(s).

But... what about the case where the guest is a different family than
hypervisor? If we're on, say, a Fam15h hypervisor but the guest is started
as a Fam0fh guest where the MSR doesn't exist and LFENCE is supposed to be
serialized?  I'll have to do a rdmsr_safe() and only set the flag(s) if I
can successfully read the MSR back and validate the bit.

Thanks,
Tom

> 
> Thanks,
> 
> 	tglx
>