linux-kernel - Re: [PATCH v3 2/9] KVM: MMU: abstract spte write-protect

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Date:	Sat, 21 Apr 2012 01:18:54 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 2/9] KVM: MMU: abstract spte write-protect

On Sat, Apr 21, 2012 at 11:24:54AM +0800, Xiao Guangrong wrote:
> On 04/21/2012 05:33 AM, Marcelo Tosatti wrote:
> 
> 
> >>  static bool
> >>  __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
> >>  {
> >> @@ -1050,24 +1078,13 @@ __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
> >>
> >>  	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
> >>  		BUG_ON(!(*sptep & PT_PRESENT_MASK));
> >> -		rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
> >> -
> >> -		if (!is_writable_pte(*sptep)) {
> >> -			sptep = rmap_get_next(&iter);
> >> -			continue;
> >> -		}
> >> -
> >> -		if (level == PT_PAGE_TABLE_LEVEL) {
> >> -			mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
> >> -			sptep = rmap_get_next(&iter);
> >> -		} else {
> >> -			BUG_ON(!is_large_pte(*sptep));
> >> -			drop_spte(kvm, sptep);
> >> -			--kvm->stat.lpages;
> > 
> > It is preferable to remove all large sptes including read-only ones, the
> 
> 
> It can cause page faults even if read memory on these large sptse.
> 
> Actually, Avi suggested that make large writable spte to be readonly
> (not dropped) on this path.

See commits e49146dce8c3dc6f4485c1904b6587855f393e71,
38187c830cab84daecb41169948467f1f19317e3 for issues
with large read-only sptes.

> > current behaviour, then to verify that no read->write transition can
> > occur in fault paths (fault paths which are increasing in number).
> 
> 
> Yes, the small spte also has issue (find a write-protected spte in
> fault paths). Later, the second part of this patchset will introduce
> rmap.WRITE_PROTECTED bit, then we can do the fast check before calling
> fast page fault.
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