linux-kernel - Re: [PATCH 2/2] KVM: MMU: fix relaxing permission

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Date:	Thu, 27 May 2010 14:18:16 +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 2/2] KVM: MMU: fix relaxing permission

On 05/27/2010 02:00 PM, Xiao Guangrong wrote:
>
>> The host always sets cr0.wp (in shadow mode) so we can write protect
>> page tables.  So when the guest clears cr0.wp, we emulate a gpte with
>> gpte.w=0 and gpte.u=1 in two ways:
>>
>> - spte.w=1, spte.u=0: this will allow the guest kernel to write but trap
>> on guest user access
>> - spte.w=0, spte.u=1: allows guest user access but traps on guest kernel
>> writes
>>
>> If the guest attempts an access that is currently disallowed, we switch
>> to the other spte encoding.
>>      
> Avi,
>
> Thanks for your explanation, but i not see where to implement what you say,
> could you please point it out for me? :-(
>    

b70ccb0b3fd removed it accidentally:

> -       } else
> -               /*
> -                * Kernel mode access.  Fail if it's a read-only page and
> -                * supervisor write protection is enabled.
> -                */
> -               if (!writable_shadow) {
> -                       if (is_write_protection(vcpu))
> -                               return 0;
> -                       *shadow_ent &= ~PT_USER_MASK;
> -               }

:(

> And, i think use 'spte.w=1, spte.u=0' to emulate 'guest cr0.wp=0 and gpte.w=0'
> is not a good way since it can completely stop user process access, but in this
> case, user process is usually read and kernel lazily to write, just like vdso,
> it will generate a lots of #PF
>    

As soon as the guest kernel stops writing we switch back to 
gpte.w=gpte.u=1 and the guest can access it completely.  For the case 
where both the kernel and userspace use interleaved access, you are 
right, but I don't see a better way, do you?


-- 
error compiling committee.c: too many arguments to function

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