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From:   Tianyu Lan <lantianyu1986@gmail.com>
Date:   Tue, 27 Aug 2019 20:33:08 +0800
Message-ID: <CAOLK0pwfxhsZN66dU2DE=qb1Y7mnVVjOCxYJP3Fi7kKgjoOFcw@mail.gmail.com>
Subject: Re: [PATCH V3 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Tianyu Lan <Tianyu.Lan@microsoft.com>, kvm <kvm@vger.kernel.org>,
        linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org,
        "linux-kernel@vger kernel org" <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>, corbet@lwn.net,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        michael.h.kelley@microsoft.com
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On Tue, Aug 27, 2019 at 8:17 PM Tianyu Lan <lantianyu1986@gmail.com> wrote:
>
> On Tue, Aug 27, 2019 at 2:41 PM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >
> > 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)).
> >
>
> Hi Vitaly::
>      Actually, I had such idea before. But user space should check
> whether hv tlb flush
> is exposed to VM before enabling direct tlb flush. If no, user space
> should not direct
> tlb flush for guest since Hyper-V will do more check for each
> hypercall from nested
> VM with enabling the feauter..
>
Fix the line break.Sorry for noise.

Actually, I had such idea before. But user space should check
whether hv tlb flush is exposed to VM before enabling direct tlb
flush. If no, user space should not direct tlb flush for guest since
Hyper-V will do more check for each hypercall from nested VM
with enabling the feauter..
---
Best regards
Tianyu Lan