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

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Date:	Tue, 4 Aug 2015 15:23:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] KVM: MMU: introduce the framework to check reserved
 bits on sptes



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.

>>
>>> +                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.

>>
>>> 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.

Paolo

> BTW, cpu_pse only hurts 32 bit page table which is not used by shadow
> page table (32 PAE and 64 Long mode are used in shadow page).
> 
> Only tdp only follows host CPU's features, KVM does not use NX to
> protect the page, so i always mark it as false in
> reset_tdp_shadow_rsvds_bits_mask().
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