Re: [PATCH V3 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support

[Tianyu Lan]

On Tue, Aug 27, 2019 at 2:41 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> lantianyu1986@xxxxxxxxx writes:
>
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > 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..

-- 
Best regards
Tianyu Lan