Re: [PATCH 6/9] KVM: MMU: introduce the framework to check reserved bits on sptes

From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: gleb@kernel.org, mtosatti@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/9] KVM: MMU: introduce the framework to check reserved bits on sptes
Date: Tue, 4 Aug 2015 21:34:38 +0800	[thread overview]
Message-ID: <55C0BF6E.2060304@linux.intel.com> (raw)
In-Reply-To: <55C0BCB4.3020601@redhat.com>

On 08/04/2015 09:23 PM, Paolo Bonzini wrote:
>
>
> On 04/08/2015 15:10, Xiao Guangrong wrote:
>>>
>>> This should be cpu_has_nx, I think.
>>
>> cpu_has_nx() checks the feature on host CPU, however, this is the shadow
>> page table which completely follow guest's features.
>>
>> E.g, if guest does not execution-protect the physical page, then
>> KVM does not do it either.
>
> That's just true for current code.  In principle you could add a memslot
> flag for KVM_MEMSLOT_NO_EXECUTE, then NX would be true on an spte but
> not on a PTE.

Yes, i agree. I would like to keep it as strict as possible to catch
potential bugs. We can relax it while KVM_MEMSLOT_NO_EXECUTE is being
developed.

>
>>>
>>>> +                guest_cpuid_has_gbpages(vcpu),
>>>
>>> This should be cpu_has_gbpages.
>>
>> E.g, if guest does not use 1G page size, it's also not used in shadow page
>> table.
>
> However, bit 7 in the shadow PDPTE is not reserved.  If you're not
> testing "is this bit reserved" but rather "should this bit be always
> zero" in the SPTE, then checking guest_cpuid is okay.  But in that case
> shadow_rsvd_check is really more like shadow_always_zero_check.

Yes, it is not reserved in hardware's point of view. shadow_always_zero_check()
seems a more meaningful name, thanks for your suggestion. :)

>
>>>
>>>> is_pse(vcpu));
>>>
>>> This should be cpu_has_pse.
>>
>> E.g, guest does no use 4M page size, then KVM does not use it either.
>
> Right, it should always be true, not cpu_has_pse, because PAE and 64-bit
> page tables always support huge (2M) pages.  Or as above, if you're
> testing "should this bit be always zero" then it's a different story.

Yeah, i will rename the function.