linux-kernel - Re: [PATCH 1/5] KVM: MMU: introduce some macros to cleanup hlist traverseing

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Message-ID: <4C039A1B.4010703@redhat.com>
Date:	Mon, 31 May 2010 14:14:35 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/5] KVM: MMU: introduce some macros to cleanup hlist
 traverseing

On 05/31/2010 05:00 AM, Xiao Guangrong wrote:
>
>>      
>>> +
>>> +#define for_each_gfn_indirect_sp(kvm, sp, gfn, pos, n)            \
>>> +  hlist_for_each_entry_safe(sp, pos, n,                    \
>>> +&kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)], hash_link)\
>>> +        if (sp->gfn == gfn&&   !sp->role.direct)
>>> +
>>> +#define for_each_gfn_indirect_valid_sp(kvm, sp, gfn, pos, n)        \
>>> +  hlist_for_each_entry_safe(sp, pos, n,                    \
>>> +&kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)], hash_link)\
>>> +        if (sp->gfn == gfn&&   !sp->role.direct&&         \
>>> +            !sp->role.invalid)
>>>
>>>        
>> Shouldn't we always skip invalid gfns?
>>      
> Actually, in kvm_mmu_unprotect_page() function, it need find out
> invalid shadow pages:
>
> |	hlist_for_each_entry_safe(sp, node, n, bucket, hash_link)
> |		if (sp->gfn == gfn&&  !sp->role.direct) {
> |			pgprintk("%s: gfn %lx role %x\n", __func__, gfn,
> |				 sp->role.word);
> |			r = 1;
> |			if (kvm_mmu_zap_page(kvm, sp))
> |				goto restart;
> |		}
>
> I'm not sure whether we can skip invalid sp here, since it can change this
> function's return value. :-(
>    

Hm.  Invalid pages don't need to be write protected.  So I think you can 
patch unprotect_page() to ignore invalid pages, and then you can convert 
it to the new macros which ignore invalid pages as well.

The invariant is: if an sp exists with !role.invalid and !unsync, then 
the page must be write protected.


>> What about providing both gfn and role to the macro?
>>
>>      
> In current code, no code simply use role and gfn to find sp,
> in kvm_mmu_get_page(), we need do other work for
> 'sp->gfn == gfn&&  sp->role != role' sp, and other functions only need compare
> some members in role, but not all members.
>    

How about just gfn?  I think everything compares against that!

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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