Re: [Xen-devel] Xen 4.3 development update

[Tim Deegan <tim@xxxxxxx>]

At 15:21 +0200 on 29 Apr (1367248894), Peter Maloney wrote:
> On 04/04/2013 07:05 PM, Tim Deegan wrote:
> > Also, if there is still a bad slowdown, caused by the p2m lookups, this
> > might help a little bit:
> >
> > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > index 38e87ce..7bd8646 100644
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -1361,6 +1361,18 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
> >          }
> >      }
> >  
> > +
> > +    /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
> > +     * a fast path for LAPIC accesses, skipping the p2m lookup. */
> > +    if ( !nestedhvm_vcpu_in_guestmode(v)
> > +         && gfn == vlapic_base_address(vcpu_vlapic(current)) >> PAGE_SHIFT 
> > )
> > +    {
> > +        if ( !handle_mmio() )
> > +            hvm_inject_hw_exception(TRAP_gp_fault, 0);
> > +        rc = 1;
> > +        goto out;
> > +    }
> > +
> >      p2m = p2m_get_hostp2m(v->domain);
> >      mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma, 
> >                                P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), 
> > NULL);
> This patch (applied to 4.2.2) has a very large improvement on my box
> (AMD FX-8150) and WinXP 32 bit.

Hmm - I expected it to be only a mild improvement.  How about this one, 
which puts in the same shortcut in another place as well?  I don't think
it will be much better than the last one, but it's worth a try.

Tim.

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index c8487b8..10b6f6b 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1361,6 +1361,17 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
         }
     }
 
+    /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
+     * a fast path for LAPIC accesses, skipping the p2m lookup. */
+    if ( !nestedhvm_vcpu_in_guestmode(v)
+         && gfn == vlapic_base_address(vcpu_vlapic(v)) >> PAGE_SHIFT )
+    {
+        if ( !handle_mmio() )
+            hvm_inject_hw_exception(TRAP_gp_fault, 0);
+        rc = 1;
+        goto out;
+    }
+
     p2m = p2m_get_hostp2m(v->domain);
     mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma, 
                               P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), NULL);
@@ -2471,6 +2482,12 @@ static enum hvm_copy_result __hvm_copy(
             gfn = addr >> PAGE_SHIFT;
         }
 
+        /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
+         * a fast path for LAPIC accesses, skipping the p2m lookup. */
+        if ( !nestedhvm_vcpu_in_guestmode(curr)
+             && gfn == vlapic_base_address(vcpu_vlapic(curr)) >> PAGE_SHIFT )
+            return HVMCOPY_bad_gfn_to_mfn;
+
         page = get_page_from_gfn(curr->domain, gfn, &p2mt, P2M_UNSHARE);
 
         if ( p2m_is_paging(p2mt) )



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