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SubjectRe: [RFC Patch 3/3] KVM/x86: Add tlb_remote_flush callback support for vmcs
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Hi Vitaly:
Thanks for your review.

On 6/12/2018 11:12 PM, Vitaly Kuznetsov wrote:
> Tianyu Lan <Tianyu.Lan@microsoft.com> writes:
>
>> Register tlb_remote_flush callback for vmcs when hyperv capability of
>> nested guest mapping flush is detected. The interface can help to reduce
>> overhead when flush ept table among vcpus for nested VM. The tradition way
>> is to send IPIs to all affected vcpus and executes INVEPT on each vcpus.
>> It will trigger several vmexits for IPI and INVEPT emulation. Hyperv provides
>> such hypercall to do flush for all vcpus.
>>
>> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
>> ---
>> arch/x86/kvm/vmx.c | 15 +++++++++++++++
>> 1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index e50beb76d846..6cb241c05690 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4737,6 +4737,17 @@ static inline void __vmx_flush_tlb(struct kvm_vcpu *vcpu, int vpid,
>> }
>> }
>>
>> +static int vmx_remote_flush_tlb(struct kvm *kvm)
>> +{
>> + struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
>> +
>> + if (!VALID_PAGE(vcpu->arch.mmu.root_hpa))
>> + return -1;
>
> Why vcpu0? Can arch.mmu.root_hpa-s differ across vCPUs? What happens if
> they do?

Yes, it may take place that arch.mmu.root_hpa is differ across vCPUs.
We may check all vcpu root_hpa and use the hypercall when there is only
one validated ept table. If not, go back to current way.

static int hv_remote_flush_tlb()
{
struct kvm_vcpu *vcpu;
u64 root_hpa = INVALID_PAGE;
int i;

kvm_for_each_vcpu(i, vcpu, kvm) {
if (!VALID_PAGE(vcpu->arch.mmu.root_hpa))
continue;

if (!VALID_PAGE(root_hpa))
root_hpa = vcpu->arch.mmu.root_hpa;
else if (root_hpa != vcpu->arch.mmu.root_hpa) {
return -1;
}
}

if (!VALID_PAGE(root_hpa))
return -1;

return hyperv_flush_guest_mapping(construct_eptp(vcpu,
root_hpa));
}


>
>> +
>> + return hyperv_flush_guest_mapping(construct_eptp(vcpu,
>> + vcpu->arch.mmu.root_hpa));
>> +}
>
> The 'vmx_remote_flush_tlb' name looks generic enough but it is actually
> Hyper-V-specific. I'd suggest renaming to something like
> hv_remote_flush_tlb().
>
>> +
>> static void vmx_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa)
>> {
>> __vmx_flush_tlb(vcpu, to_vmx(vcpu)->vpid, invalidate_gpa);
>> @@ -7495,6 +7506,10 @@ static __init int hardware_setup(void)
>> if (enable_ept && !cpu_has_vmx_ept_2m_page())
>> kvm_disable_largepages();
>>
>> + if (ms_hyperv.nested_features & HV_X64_NESTED_GUSET_MAPPING_FLUSH
>> + && enable_ept)
>> + kvm_x86_ops->tlb_remote_flush = vmx_remote_flush_tlb;
>> +
>> if (!cpu_has_vmx_ple()) {
>> ple_gap = 0;
>> ple_window = 0;
>
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