linux-kernel - Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

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Message-ID: <f9735fb8-650e-c263-36a7-61390ccbb662@redhat.com>
Date:   Wed, 16 Oct 2019 18:38:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock

On 16/10/19 18:25, Xiaoyao Li wrote:
>>
>>    3 | Y         |     Y       |  N  |   Y     |   x   | Switch
>> MSR_TEST_CTRL on
>>      |           |             |     |         |       | enter/exit,
>> plus:
>>      |           |             |     |         |       | A) #AC
>> forwarded to guest.
>>      |           |             |     |         |       | B) SIGBUS or
>> KVM exit code
>>
> 
> I just want to get confirmed that in (3), we should split into 2 case:
> 
> a) if host has it enabled, still apply the constraint that guest is
> forcibly enabled? so we don't switch MSR_TEST_CTL.
> 
> b) if host has it disabled, we can switch MSR_TEST_CTL on enter/exit.

That's doable, yes.

Paolo