Re: [PATCH] KVM: vmx: hyper-v: don't pass EPT configuration info to vmx_hv_remote_flush_tlb()

[Vitaly Kuznetsov]

Tianyu Lan <lantianyu1986@xxxxxxxxx> writes:

> On Thu, Oct 11, 2018 at 8:18 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>>
>> Tianyu Lan <lantianyu1986@xxxxxxxxx> writes:
>>
>> > On Thu, Oct 11, 2018 at 6:32 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>> >>
>> >> I'm observing random crashes in multi-vCPU L2 guests running on KVM on
>> >> Hyper-V. I bisected the issue to the commit 877ad952be3d ("KVM: vmx: Add
>> >> tlb_remote_flush callback support"). Hyper-V TLFS states:
>> >>
>> >> "AddressSpace specifies an address space ID (an EPT PML4 table pointer)"
>> >>
>> >> So apparently, Hyper-V doesn't expect us to pass naked EPTP, only PML4
>> >> pointer should be used. Strip off EPT configuration information before
>> >> calling into vmx_hv_remote_flush_tlb().
>> >
>> > Hi Vitaly:
>> > :               Thanks to fix this. Sorry, I didn't meet the issue..
>> > I think we may just store EPT PML4 table pointer without EPT
>> > configuration information
>> > in the ept_pointer field for this case.
>> >
>> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> > index 619307b3e6bb..e316058b41a6 100644
>> > --- a/arch/x86/kvm/vmx.c
>> > +++ b/arch/x86/kvm/vmx.c
>> > @@ -5379,7 +5379,7 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu,
>> > unsigned long cr3)
>> >
>> >                 if (kvm_x86_ops->tlb_remote_flush) {
>> >                         spin_lock(&to_kvm_vmx(kvm)->ept_pointer_lock);
>> > -                       to_vmx(vcpu)->ept_pointer = eptp;
>> > +                       to_vmx(vcpu)->ept_pointer = cr3;
>>
>> True, we can do that (and I even had a version of my patch doing so)
>> but 'ept_pointer' will likely need to be renamed as it's not obvious
>> why vcpu->ept_pointer != eptp.
>>
>
> Yes. that need to rename ept_pointer.
>

Honestly, I would prefer to keep more information cached, e.g. if
someone needs EPT configuration data later he can easily get it from
ept_pointer and by putting raw cr3 there we'll just keep less.

But I don't have a strong opinion, I'll leave it up to the maintainers
to tell us how to proceed)

>> Alternatively, we can filter lower 12 bits out in
>> hyperv_flush_guest_mapping() but I would rename 'as' parameter to eptp
>> then.
>
> OK. I got it. Thanks.

-- 
Vitaly