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

[Avi Kivity <avi@redhat.com>]

On 05/27/2010 02:18 PM, Avi Kivity wrote:
>
>> 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?

To expand, we only set spte.w=1 on write faults.  So if the guest only 
reads the page, we'll instantiate an spte with u=1 and w=0.

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