linux-kernel - Re: [PATCH v1 3/4] KVM: MMU: Add 5 level EPT & Shadow page table support.

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Message-ID: <d907d814-d346-15e8-e281-f180f0fce1eb@redhat.com>
Date:   Mon, 14 Aug 2017 16:13:35 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        xiaoguangrong@...cent.com, joro@...tes.org
Subject: Re: [PATCH v1 3/4] KVM: MMU: Add 5 level EPT & Shadow page table
 support.

On 14/08/2017 13:37, Yu Zhang wrote:
> Thanks a lot for your comments, Paolo. :-)
> 
> 
> On 8/14/2017 3:31 PM, Paolo Bonzini wrote:
>> On 12/08/2017 15:35, Yu Zhang wrote:
>>>   struct rsvd_bits_validate {
>>> -    u64 rsvd_bits_mask[2][4];
>>> +    u64 rsvd_bits_mask[2][5];
>>>       u64 bad_mt_xwr;
>>>   };
>>
>> Can you change this 4 to PT64_ROOT_MAX_LEVEL in patch 2?
> 
> Well, I had tried, but failed to find a neat approach to do so. The
> difficulty I have met is that PT64_ROOT_MAX_LEVEL is defined together
> with PT64_ROOT_4LEVEL/PT32E_ROOT_LEVEL/PT32_ROOT_LEVEL in mmu.h, yet
> the rsvd_bits_validate structure is defined in kvm_host.h, which are 
> included in quite a lot .c files that do not include mmu.h or include
> the mmu.h after kvm_host.h.
> 
> I guess that's the reason why the magic number 4 instead of
> PT64_ROOT_4LEVEL is used in current definition of rsvd_bits_vadlidate. :-)

Yes, you're right.  I think the solution is to define
PT64_ROOT_MAX_LEVEL in kvm_host.h.

>>> @@ -4444,7 +4457,7 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu
>>> *vcpu, bool execonly,
>>>         MMU_WARN_ON(VALID_PAGE(context->root_hpa));
>>>   -    context->shadow_root_level = kvm_x86_ops->get_tdp_level();
>>> +    context->shadow_root_level = kvm_x86_ops->get_tdp_level(vcpu);
>>>         context->nx = true;
>>>       context->ept_ad = accessed_dirty;
>> Below, there is:
>>
>>          context->root_level = context->shadow_root_level;
>>
>> this should be forced to PT64_ROOT_4LEVEL until there is support for
>> nested EPT 5-level page tables.
> 
> So the context->shadow_root_level could be 5 or 4, and
> context->root_level is always 4?

That was my idea, but setting both to 4 should be fine too as you
suggest below.

> My understanding is that shadow ept level should be determined by
> the width of ngpa, and that if L1 guest is not exposed with EPT5
> feature, it shall only use 4 level ept for L2 guest, and the shadow
> ept does not need a 5 level one. Is this understanding correct? And
> how about we set both values to PT64_ROOT_4LEVEL for now?>
> Besides, if we wanna support nested EPT5, what do you think we need to
> do besides exposing the EPT5 feature to L1 guest?

Nothing else, I think.

Paolo