Tianyu Lan [off-list ref] writes:

On Tue, Aug 27, 2019 at 2:41 PM Vitaly Kuznetsov [off-list ref] wrote:

quoted

lantianyu1986@gmail.com writes:

quoted

From: Tianyu Lan <redacted>

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..

If TLB Flush enlightenment is not exposed to the VM at all there's no
difference if we enable direct TLB flush in eVMCS or not: the guest
won't be using 'TLB Flush' hypercall and will do TLB flushing with
IPIs. And, in case the guest enables Hyper-V hypercall page, it is
definitelly not going to use KVM hypercalls so we can't break these.

-- 
Vitaly