Re: [PATCH V4 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support

From: Tianyu Lan <lantianyu1986@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	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>,
	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
Subject: Re: [PATCH V4 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support
Date: Wed, 18 Sep 2019 09:55:34 +0800	[thread overview]
Message-ID: <CAOLK0pzVGT2hV=OgJf0pJ5ih2AHN9N8pk1WC38835A-WZD0EMA@mail.gmail.com> (raw)
In-Reply-To: <874l1baqnf.fsf@vitty.brq.redhat.com>

On Tue, Sep 17, 2019 at 11:28 PM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Paolo Bonzini <pbonzini@redhat.com> writes:
>
> > On 22/08/19 16:30, lantianyu1986@gmail.com wrote:
> >> 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"
> >>
> >> Change since v3:
> >>        - Update changelog in each patches.
> >>
> >> Change since v2:
> >>        - Move hv assist page(hv_pa_pg) from struct kvm  to struct kvm_hv.
> >>
> >> Change since v1:
> >>        - Fix offset issue in the patch 1.
> >>        - Update description of KVM KVM_CAP_HYPERV_DIRECT_TLBFLUSH.
> >>
> >> Tianyu Lan (2):
> >>   x86/Hyper-V: Fix definition of struct hv_vp_assist_page
> >>   KVM/Hyper-V: Add new KVM capability KVM_CAP_HYPERV_DIRECT_TLBFLUSH
> >>
> >> Vitaly Kuznetsov (1):
> >>   KVM/Hyper-V/VMX: Add direct tlb flush support
> >>
> >>  Documentation/virtual/kvm/api.txt  | 13 +++++++++++++
> >>  arch/x86/include/asm/hyperv-tlfs.h | 24 ++++++++++++++++++-----
> >>  arch/x86/include/asm/kvm_host.h    |  4 ++++
> >>  arch/x86/kvm/vmx/evmcs.h           |  2 ++
> >>  arch/x86/kvm/vmx/vmx.c             | 39 ++++++++++++++++++++++++++++++++++++++
> >>  arch/x86/kvm/x86.c                 |  8 ++++++++
> >>  include/uapi/linux/kvm.h           |  1 +
> >>  7 files changed, 86 insertions(+), 5 deletions(-)
> >>
> >
> > Queued, thanks.
> >
>
> I had a suggestion how we can get away without the new capability (like
> direct tlb flush gets automatically enabled when Hyper-V hypercall page
> is activated and we know we can't handle KVM hypercalls any more)
> but this can probably be done as a follow-up.
>

Hi Vital'y：
              Actually, I have tried your proposal but it turns out
KVM in L1 fails to
enable direct tlb flush most time after nested VM starts.
"hv_enlightenments_control.
nested_flush_hypercall" flag in evmcs is cleared by Hyper-V after run
nested VM. I still
wait answer from Hyper-V team. So far, it looks like enabling direct
tlb flush before start
nested VM is a safe way.Once get more infomration from Hyper-V team and we may
have a look to how to enable your proposal.
-- 
Best regards
Tianyu Lan