RE: [Xen-ia64-devel] [PATCH] xen might misunderstand a normal page asI/O page

[Kouya SHIMURA <kouya@xxxxxxxxxxxxxx>]

Hi Anthony,

The guest OS can use ig field{63:53} in VHPT Short/Long Format.
Actually Windows seems to use this field and sometimes set bit{60}.
Xen/IPF also uses bit{60} of PTE as VTLB_PTE_IO_BIT. 
So misunderstanding may happen.

When first TLB-miss happens, the pteval in a guest VHPT propagates as
follows:

In vmx_hpw_miss(),
 => }else if(type == DSIDE_TLB){
   => if (!guest_vhpt_lookup(vhpt_adr, &pteval)) {
     => thash_purge_and_insert(v, pteval, itir, vadr, DSIDE_TLB);

In thash_purge_and_insert(),
 => if(VMX_DOMAIN(v)){
   =>   if (ps == mrr.ps) {
     =>     if(!(pte&VTLB_PTE_IO)){ <<<<< This condition is failure
            else{
                vtlb_insert(v, pte, itir, ifa);
                vcpu_quick_region_set(PSCBX(v,tc_regions),ifa);
            }

After all, this TLB-miss wastes a vtlb and reproduce TLB-miss again.
When the second TLB-miss happens, 

In vmx_hpw_miss(),
 => if((data=vtlb_lookup(v, vadr,type))!=0){
   =>   if (v->domain != dom0 && type == DSIDE_TLB) {
     =>     if (__gpfn_is_io(v->domain, gppa >> PAGE_SHIFT)) { <<<<< failure. 
resolve misunderstanding
     => thash_vhpt_insert(v, data->page_flags, data->itir, vadr, type);

My patch just masks the ig field. 

Thanks
Kouya

Xu, Anthony writes:
 > Hi Kouya,
 > 
 > Can you explain more?
 > 
 > How does misunderstanding happen?
 > And how does this patch fix it?
 > 
 > Thanks
 > Anthony
 > 
 > Kouya SHIMURA write on 2007年1月24日 12:31:
 > > Hi,
 > > 
 > > Hypervisor might misunderstand a normal page as I/O page
 > > if a guest OS uses the ig field in the guest VHPT.
 > > 
 > > It seems to be harmless but slightly slow down.
 > > 
 > > Thanks,
 > > Kouya
 > > 
 > > Signed-off-by: Kouya Shimura <kouya@xxxxxxxxxxxxxx>


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