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

[Xiao Guangrong]

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!