linux-kernel - Re: [PATCH 3/4] KVM: MMU: track dirty page in speculative path properly

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Date:	Wed, 14 Jul 2010 17:25:05 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/4] KVM: MMU: track dirty page in speculative path properly

On 07/14/2010 05:06 PM, Marcelo Tosatti wrote:
>
>> Yeah, you are right, and i want to use another way to do it since track dirty bit
>> is too heavy, also it's dangerous if we miss to set page dirty.
>>
>> How about just track access bit for speculative path, we set page both accessed and
>> dirty(if it's writable) only if the access bit is set?
>>      
> A useful thing to do would be to allow read-only mappings, in the fault
> path (Lai sent a few patches in that direction sometime ago but there
> was no follow up).
>
> So in the case of a read-only fault from the guest, you'd inform
> get_user_pages() that read-only access is acceptable (so swapcache pages
> can be mapped, or qemu can mprotect(PROT_READ) guest memory).
>    

I'd like get_user_pages_ptes_fast(), that returns the pte along with the 
page.  It can be used for a couple of purposes:

- on read faults or speculative mappings, ask for read access, but allow 
write if the pte is writeable
- stick the page size into  free bits (need 2 to support 1G pages) to 
speed up host_mapping_level()

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

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