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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     lantianyu1986@gmail.com
Cc:     Tianyu Lan <Tianyu.Lan@microsoft.com>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org,
        linux-kernel@vger.kernel.org, pbonzini@redhat.com,
        rkrcmar@redhat.com, corbet@lwn.net, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, sashal@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        x86@kernel.org, michael.h.kelley@microsoft.com
Subject: Re: [PATCH V3 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support
In-Reply-To: <20190819131737.26942-1-Tianyu.Lan@microsoft.com>
References: <20190819131737.26942-1-Tianyu.Lan@microsoft.com>
Date:   Tue, 27 Aug 2019 08:41:43 +0200
Message-ID: <87ftlnm7o8.fsf@vitty.brq.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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lantianyu1986@gmail.com writes:

> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
>
> This patchset is to add Hyper-V direct tlb support in KVM. Hyper-V
> in L0 can delegate L1 hypervisor to handle tlb flush request from
> L2 guest when direct tlb flush is enabled in L1.
>
> Patch 2 introduces new cap KVM_CAP_HYPERV_DIRECT_TLBFLUSH to enable
> feature from user space. User space should enable this feature only
> when Hyper-V hypervisor capability is exposed to guest and KVM profile
> is hided. There is a parameter conflict between KVM and Hyper-V hypercall.
> We hope L2 guest doesn't use KVM hypercall when the feature is
> enabled. Detail please see comment of new API
> "KVM_CAP_HYPERV_DIRECT_TLBFLUSH"

I was thinking about this for awhile and I think I have a better
proposal. Instead of adding this new capability let's enable direct TLB
flush when KVM guest enables Hyper-V Hypercall page (writes to
HV_X64_MSR_HYPERCALL) - this guarantees that the guest doesn't need KVM
hypercalls as we can't handle both KVM-style and Hyper-V-style
hypercalls simultaneously and kvm_emulate_hypercall() does:

	if (kvm_hv_hypercall_enabled(vcpu->kvm))
		return kvm_hv_hypercall(vcpu);

What do you think?

(and instead of adding the capability we can add kvm.ko module parameter
to enable direct tlb flush unconditionally, like
'hv_direct_tlbflush=-1/0/1' with '-1' being the default (autoselect
based on Hyper-V hypercall enablement, '0' - permanently disabled, '1' -
permanenetly enabled)).

-- 
Vitaly