linux-kernel - Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

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Message-ID: <a6e9867c-15f0-96be-04fa-456cbe826ffb@intel.com>
Date:   Thu, 16 Apr 2020 20:34:37 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

On 4/16/2020 5:22 AM, Thomas Gleixner wrote:
> Sean,
> 
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
>> On Wed, Apr 15, 2020 at 07:43:22PM +0200, Thomas Gleixner wrote:
>>> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>>>> +static inline void vmx_update_sld(struct kvm_vcpu *vcpu, bool on)
>>>> +{
>>>> +	/*
>>>> +	 * Toggle SLD if the guest wants it enabled but its been disabled for
>>>> +	 * the userspace VMM, and vice versa.  Note, TIF_SLD is true if SLD has
>>>> +	 * been turned off.  Yes, it's a terrible name.
>>>
>>> Instead of writing that useless blurb you could have written a patch
>>> which changes TIF_SLD to TIF_SLD_OFF to make it clear.
>>
>> Hah, that's my comment, though I must admit I didn't fully intend for the
>> editorial at the end to get submitted upstream.
>>
>> Anyways, I _did_ point out that TIF_SLD is a terrible name[1][2], and my
>> feedback got ignored/overlooked.  I'd be more than happy to write a
>> patch, I didn't do so because I assumed that people wanted TIF_SLD as the name for
>> whatever reason.
> 
> I somehow missed that in the maze of mails regarding this stuff. I've
> already written a patch to rename it to TIF_SLD_DISABLED which is pretty
> self explaining. But see below.
> 
[...]
> 
>>>> @@ -1188,6 +1217,10 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>>>>   #endif
>>>>
>>>> 	vmx_set_host_fs_gs(host_state, fs_sel, gs_sel, fs_base, gs_base);
>>>> +
>>>> +	vmx->host_sld_on = !test_thread_flag(TIF_SLD);
>>>
>>> This inverted storage is non-intuitive. What's wrong with simply
>>> reflecting the TIF_SLD state?
>>
>> So that the guest/host tracking use the same polairy, and IMO it makes
>> the restoration code more intuitive, e.g.:
>>
>> 	vmx_update_sld(&vmx->vcpu, vmx->host_sld_on);
>> vs
>> 	vmx_update_sld(&vmx->vcpu, !vmx->host_tif_sld);
>>
>> I.e. the inversion needs to happen somewhere.
> 
> Correct, but we can make it consistently use the 'disabled' convention.
> 
> I briefly thought about renaming the flag to TIF_SLD_ENABLED, set it by
> default and update the 5 places where it is used. But that's
> inconsistent as well simply because it does not make any sense to set
> that flag when detection is not available or disabled on the command
> line.
> 

Assuming you'll pick TIF_SLD_DISABLED, I guess we need to set this flag 
by default for the case SLD is no available or disabled on the command, 
for consistency?