linux-kernel - Re: [PATCH] KVM: vmx: hyper-v: don't pass EPT configuration info to vmx_hv_remote_flush

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Date:   Thu, 11 Oct 2018 14:18:19 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tianyu Lan <lantianyu1986@...il.com>
Cc:     kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        Michael.H.Kelley@...rosoft.com,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-kernel\@vger kernel org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: vmx: hyper-v: don't pass EPT configuration info to vmx_hv_remote_flush_tlb()

Tianyu Lan <lantianyu1986@...il.com> writes:

> On Thu, Oct 11, 2018 at 6:32 PM Vitaly Kuznetsov <vkuznets@...hat.com> 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.

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

[snip]

-- 
Vitaly