Re: [Xen-devel] PVH and mtrr/PAT.........

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Wed, 20 Nov 2013 18:12:13 +0000
George Dunlap <dunlapg@xxxxxxxxx> wrote:

> On Wed, Nov 20, 2013 at 8:42 AM, Jan Beulich <JBeulich@xxxxxxxx>
> wrote:
> >>>> On 20.11.13 at 03:11, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
> >>>> wrote:
> >> After rebasing my dom0 on latest, it didn't boot. After debugging
> >> couple days, it turned out to be :
> >>
> >> +    if ( is_pvh_domain(d) )
> >> +    {
> >> +        if ( direct_mmio )
> >> +            return MTRR_TYPE_UNCACHABLE;
> >> +        return MTRR_TYPE_WRBACK;
> >> +    }
> >> +
> >>
> >> I had in my patches, missing in epte_get_entry_emt() in latest.
> >>
> >> So, since I don't know much about this, is an HVM guest setting
> >> MTRR range types? Looking for suggestions on best way to do this
> >> for PVH.
> >
> > A HVM guest is permitted to write to (virtual) MTRRs, whereas a PV
> > guest isn't. I'm inclined to prefer PV behavior here to be used for
> > PVH (since, as explained by Dongxiao, MTRRs don't really matter
> > for VMX guests anyway, i.e. the setting of (virtual) MTRRs needs to
> > get translated to EPT memory types anyway, hence a PVH guest
> > ought to be fine ignoring the MTRRs altogether and handling memory
> > types exclusively via PAT mechanisms).
> 
> Mukesh,
> 
> Do you know why this line is having this effect?  For one, is it a
> matter of direct_mmio pages being given something other than
> UNCACHEABLE, or a matter of non-direct_mmio pages given something
> other than WRBACK?
> 
> And is the problem that the guest is *not* setting MTRRs, or that the
> guest *is* setting MTRRs?
> 
> I'd prefer to avoid having a special case for PVH in this path if
> possible.

Without any changes to epte_get_entry_emt(), all types are being returned 
as MTRR_TYPE_WRBACK for PVH because of:

    if ( !v->domain->arch.hvm_domain.params[HVM_PARAM_IDENT_PT] )
            return MTRR_TYPE_WRBACK;

This is problem for low level non-ram pages being accessed in dom0,
(which interesting gave MCE errors). non-ram IO pages have to be
MTRR_TYPE_UNCACHABLE.

After changing this to, 
    if ( !is_pvh_vcpu(v) &&
         !v->domain->arch.hvm_domain.params[HVM_PARAM_IDENT_PT] )

I started hitting if ( direct_mmio ), and getting proper UNCACHABLE
for io pages, but RAM pages started being returned as UNCACHABLE also 
thru get_mtrr_type() which I've not investigated.

For domU, it's incorrect, but happens to work because of:

    if ( !v->domain->arch.hvm_domain.params[HVM_PARAM_IDENT_PT] )
            return MTRR_TYPE_WRBACK;

as domU only has RAM pages, and thus WRBACK is correct for all.

My quick fix while we come up with better solution was:
-----------
+    /* PVH fixme: Add support for more memory types. */
+    if ( is_pvh_domain(d) )
+    {
+        if ( direct_mmio )
+            return MTRR_TYPE_UNCACHABLE;
+        return MTRR_TYPE_WRBACK;
+    }
+
     if ( !v->domain->arch.hvm_domain.params[HVM_PARAM_IDENT_PT] )
---------------

It appears you didn't check all places where params was being used
before adding it for PVH.

thanks
Mukesh

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