linux-kernel - Re: [PATCH 3/5] mmu: don't set the present bit unconditionally

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Date:	Tue, 5 Jul 2016 13:50:39 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Bandan Das <bsd@...hat.com>, kvm <kvm@...r.kernel.org>,
	guangrong.xiao@...ux.intel.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] mmu: don't set the present bit unconditionally

2016-06-28 16:57 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 28/06/2016 06:32, Bandan Das wrote:
>> +     bool execonly = !(context->guest_rsvd_check.bad_mt_xwr &
>> +                       (1ull << VMX_EPT_EXECUTABLE_MASK));
>>
>>       if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
>>               return 0;
>>
>> -     spte = PT_PRESENT_MASK;
>> +     if (!execonly)
>> +             spte |= PT_PRESENT_MASK;
>
> This needs a comment:
>
>         /*
>          * There are two cases in which execonly is false: 1) for
>          * non-EPT page tables, in which case we need to set the
>          * P bit; 2) for EPT page tables where an X-- page table

In the scenario of non-EPT shadow page table and non-nested, the
present bit can't be set any more since
context->guest_rsvd_check.bad_mt_xwr is always 0.

Regards,
Wanpeng Li