linux-kernel - Re: [PATCH v3 5/6] KVM: MMU: abstract invalid guest pte mapping

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Date:	Mon, 22 Nov 2010 18:18:21 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, KVM <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/6] KVM: MMU: abstract invalid guest pte mapping


On 11/22/2010 05:28 PM, Avi Kivity wrote:

>> +static bool FNAME(map_invalid_gpte)(struct kvm_vcpu *vcpu,
>> +                    struct kvm_mmu_page *sp, u64 *spte,
>> +                    pt_element_t gpte)
> 
> It's really only for speculative maps, the name should reflect that.
> 

OK, i'll use speculative_map_invalid_gpte or speculative_map_gpte
instead.

> Why restrict to invalid gptes?  Won't it work for valid gptes as well? 
> Maybe you'll need an extra code path for update_pte() which already
> knows the pfn.
> 

Um. i did it in the in the previous version, but it needs a callback to
get pfn since get pfn is very different on update_pte / prefetch_pte /
sync_page paths. the codes seems more complicated.

Maybe we can get pfn first and call FNAME(map_vaild_gpte) later, but
it can add little little overload on prefetch_pte path.

>> +{
>> +    u64 nonpresent = shadow_trap_nonpresent_pte;
>> +
>> +    if (is_rsvd_bits_set(&vcpu->arch.mmu, gpte, PT_PAGE_TABLE_LEVEL))
>> +        goto no_present;
>> +
>> +    if (!is_present_gpte(gpte)) {
>> +        if (!sp->unsync)
>> +            nonpresent = shadow_notrap_nonpresent_pte;
>> +        goto no_present;
>> +    }
> 
> I think the order is reversed.  If !is_present_gpte(), it doesn't matter
> if reserved bits are set or not.
> 

if !is_present_gpte() && is_rsvd_bits_set, then we may mark the spte notrap,
so the guest will detect #PF with PFEC.P=PEFC.RSVD=0, but the appropriate PFEC
is PFEC.P=0 && PEFC.RSVD=1 ?
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