Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

Hi Marcelo,

Thanks for your patience. I was reading your reply over and over again, i would
like to argue it more :).
Please see below.

On 11/29/2012 08:21 AM, Marcelo Tosatti wrote:

> 
> https://lkml.org/lkml/2012/11/17/75
> 
> Does unshadowing work with large sptes at reexecute_instruction? That
> is, do we nuke any large read-only sptes that might be causing a certain
> gfn to be read-only?
> 
> That is, following the sequence there, is the large read-only spte
> properly destroyed?

Actually, sptes can not prevent gfn becoming writable, that means the gfn
can become writable *even if* it exists a spte which maps to the gfn.

The condition that can prevent gfn becoming writable is, the gfn has been
shadowed, that means the gfn can not become writable if it exists a sp
with sp.gfn = gfn.

Note, drop_spte does not remove any sp. So, either destroying spte or keeping
spte doest not have any affect for gfn write-protection.

If luck enough, my point is right, the current code can do some optimizations
as below:

                if (level > PT_PAGE_TABLE_LEVEL &&
-                   has_wrprotected_page(vcpu->kvm, gfn, level)) {
-                       ret = 1;
-                       drop_spte(vcpu->kvm, sptep);
-                       goto done;
-               }
+                   has_wrprotected_page(vcpu->kvm, gfn, level))
+                       return 0;


1): we can return 0 instead of 1 to avoid unnecessary emulation. vcpu will refault
    again then kvm will use small page.

2): need not do any change on the spte.