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Subject: Re: [PATCH v2 1/3] KVM: x86: assign two bits to track SPTE kinds
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Ben Gardon <bgardon@google.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, Vitaly Kuznetsov <vkuznets@redhat.com>,
        Junaid Shahid <junaids@google.com>
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 <20191211191327.GI5044@linux.intel.com>
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 <20191212002902.GM5044@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5c39ee9b-3b58-f1c9-889d-6e012bbbac6c@redhat.com>
Date:   Thu, 12 Dec 2019 01:33:45 +0100
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On 12/12/19 01:29, Sean Christopherson wrote:
>>   */
>> -#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(18, 0)
>> +#define MMIO_SPTE_GEN_MASK		GENMASK_ULL(17, 0)
>>  
>>  #define MMIO_SPTE_GEN_LOW_START		3
>>  #define MMIO_SPTE_GEN_LOW_END		11
>>  #define MMIO_SPTE_GEN_LOW_MASK		GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
>>  						    MMIO_SPTE_GEN_LOW_START)
>>  
>> -#define MMIO_SPTE_GEN_HIGH_START	52
>> -#define MMIO_SPTE_GEN_HIGH_END		61
>> +/* Leave room for SPTE_SPECIAL_MASK.  */
>> +#define MMIO_SPTE_GEN_HIGH_START	PT64_SECOND_AVAIL_BITS_SHIFT
> I'd rather have GEN_HIGH_START be an explicit bit number and then add
> a BUILD_BUG_ON(GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT) to ensure
> the MMIO gen doesn't overlap other stuff.  That way we get a build error
> if someone changes PT64_SECOND_AVAIL_BITS_SHIFT, otherwise the MMIO gen
> will end up who knows where and probably overwrite NX or EPT.SUPPRESS_VE.
> 

Fair enough.  While at it I'll also add MMIO_SPTE_GEN_BITS (
	MMIO_SPTE_GEN_LOW_END - MMIO_SPTE_GEN_LOW_START + 1
	+ MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1) and use it in
MMIO_SPTE_GEN_MASK.

Paolo