linux-kernel - Re: [PATCH v2 04/22] x86/kvm: Don't call kvm_spurious

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Date:   Thu, 18 Jul 2019 16:13:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 04/22] x86/kvm: Don't call kvm_spurious_fault() from
 .fixup

On 18/07/19 16:12, Josh Poimboeuf wrote:
> On Thu, Jul 18, 2019 at 03:18:50PM +0200, Paolo Bonzini wrote:
>> On 18/07/19 15:16, Sean Christopherson wrote:
>>>> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
>>>>
>>>> This has a side effect of adding a jump in a generally hot path, but
>>>> let's hope that the speculation gods for once help us.
>>> Any reason not to take the same approach as vmx_vmenter() and ud2 directly
>>> from fixup?  I've never found kvm_spurious_fault() to be all that helpful,
>>> IMO it's a win win. :-)
>>
>> Honestly I've never seen a backtrace from here but I would rather not
>> regret this when a customer encounters it...
> 
> In theory, changing the "call kvm_spurious_fault" to ud2 should be fine.
> It should be tested, of course.
> 
> I would defer to Sean to make the patch on top of mine :-)
> 

Yes, this can be done easily on top.

Paolo