linux-kernel - Re: [PATCH 0/7] KVM: MMU: fast write protect

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Date:   Thu, 4 May 2017 11:36:55 +0800
From:   Xiao Guangrong <guangrong.xiao@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, mtosatti@...hat.com,
        avi.kivity@...il.com, rkrcmar@...hat.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, Xiao Guangrong <xiaoguangrong@...cent.com>
Subject: Re: [PATCH 0/7] KVM: MMU: fast write protect



On 05/03/2017 10:57 PM, Paolo Bonzini wrote:
> 
> 
> On 03/05/2017 16:50, Xiao Guangrong wrote:
>> Furthermore, userspace has no knowledge about if PML is enable (it
>> can be required from sysfs, but it is a good way in QEMU), so it is
>> difficult for the usespace to know when to use write-protect-all.
>> Maybe we can make KVM_CAP_X86_WRITE_PROTECT_ALL_MEM return false if
>> PML is enabled?
> 
> Yes, that's a good idea.  Though it's a pity that, with PML, setting the
> dirty bit will still do the massive walk of the rmap.  At least with
> reset_dirty_pages it's done a little bit at a time.
> 
>>> Also, I wonder how the alternative write protection mechanism would
>>> affect performance of the dirty page ring buffer patches.  You would do
>>> the write protection of all memory at the end of
>>> kvm_vm_ioctl_reset_dirty_pages.  You wouldn't even need a separate
>>> ioctl, which is nice.  On the other hand, checkpoints would be more
>>> frequent and most pages would be write-protected, so it would be more
>>> expensive to rebuild the shadow page tables...
>>
>> Yup, write-protect-all can improve reset_dirty_pages indeed, i will
>> apply your idea after reset_dirty_pages is merged.
>>
>> However, we still prefer to have a separate ioctl for write-protect-all
>> which cooperates with KVM_GET_DIRTY_LOG to improve live migration that
>> should not always depend on checkpoint.
> 
> Ok, I plan to merge the dirty ring pages early in 4.13 development.

Great.

As there is no conflict between these two patchsets except dirty
ring pages takes benefit from write-protect-all, i think they
can be developed and iterated independently, right?

Or you prefer to merge dirty ring pages first then review the
new version of this patchset later?

Thanks!