linux-kernel - Re: [PATCH v2 3/7] KVM: x86: hyper-v: Move the remote TLB flush logic out of vmx

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Date:   Tue, 20 Apr 2021 17:57:19 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Vineeth Pillai <viremana@...ux.microsoft.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org,
        Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Wei Liu <wei.liu@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        viremana@...ux.microsoft.com
Subject: Re: [PATCH v2 3/7] KVM: x86: hyper-v: Move the remote TLB flush
 logic out of vmx

Vineeth Pillai <viremana@...ux.microsoft.com> writes:

> On 4/16/2021 4:36 AM, Vitaly Kuznetsov wrote:
>>
>>>   struct kvm_vm_stat {
>>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>>> index 58fa8c029867..614b4448a028 100644
>>> --- a/arch/x86/kvm/hyperv.c
>>> +++ b/arch/x86/kvm/hyperv.c
>> I still think that using arch/x86/kvm/hyperv.[ch] for KVM-on-Hyper-V is
>> misleading. Currently, these are dedicated to emulating Hyper-V
>> interface to KVM guests and this is orthogonal to nesting KVM on
>> Hyper-V. As a solution, I'd suggest you either:
>> - Put the stuff in x86.c
>> - Create a dedicated set of files, e.g. 'kvmonhyperv.[ch]' (I also
>> thought about 'hyperv_host.[ch]' but then I realized it's equally
>> misleading as one can read this as 'KVM is acting as Hyper-V host').
>>
>> Personally, I'd vote for the later. Besides eliminating confusion, the
>> benefit of having dedicated files is that we can avoid compiling them
>> completely when !IS_ENABLED(CONFIG_HYPERV) (#ifdefs in C are ugly).
> Makes sense, creating new set of files looks good to me. The default 
> hyperv.c
> for hyperv emulation also seems misleading - probably we should rename it
> to hyperv_host_emul.[ch] or similar. That way, probably I can use 
> hyperv.[ch]
> for kvm on hyperv code. If you feel, thats too big of a churn, I shall use
> kvm_on_hyperv.[ch] (to avoid reading the file differently). What do you 
> think?

I agree that 'hyperv.[ch]' is not ideal but I'm on the fence whether
renaming it is worth it. If we were to rename it, I'd suggest just
'hyperv_emul.[ch]' to indicate that here we're emulating Hyper-V.

I don't think reusing 'hyperv.[ch]' for KVM-on-Hyper-V is a good idea,
it would be doubly misleading and not friendly to backporters. Let's not
do that.

>
>
>>> @@ -10470,7 +10474,6 @@ void kvm_arch_free_vm(struct kvm *kvm)
>>>   	vfree(kvm);
>>>   }
>>>   
>>> -
>> Stray change?
> It was kinda leftover, but I thought I'd keep it as it removes and 
> unnecessary line.

The idea is to have meaninful patches as concise as possible splitting
off cleanup / preparatory patches which don't actually change anything;
this way big series are much easier to review.

>
> Thanks,
> Vineeth
>

-- 
Vitaly