Re: [Xen-devel] [V1 PATCH] PVH: avoid call to handle_mmio

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Thu, 05 Jun 2014 07:28:30 +0100
"Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> >>> On 05.06.14 at 01:52, <mukesh.rathor@xxxxxxxxxx> wrote:
> > On Wed, 04 Jun 2014 08:24:15 +0100
> > "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> > 
> >> >>> On 04.06.14 at 00:00, <mukesh.rathor@xxxxxxxxxx> wrote:
> >> > handle_mmio() is currently unsafe for pvh guests. A call to it
> >> > would result in call to vioapic_range that will crash xen since
> >> > the vioapic ptr in struct hvm_domain is not initialized for pvh
> >> > guests.
> >> > 
> >> > However, one path exists for such a call. If a pvh guest, dom0 or
> >> > domU, unintentionally touches non-existing memory, an EPT
> >> > violation would occur. This would result in unconditional call to
> >> > hvm_hap_nested_page_fault. In that function, because
> >> > get_gfn_type_access returns p2m_mmio_dm for non existing mfns by
> >> > default, handle_mmio() will get called. This would result in xen
> >> > crash instead of the guest crash. This patch addresses that.
> >> 
> >> Yes, we definitely want this until being properly handled, no
> >> matter that crashing the guest here doesn't seem to be the right
> >> thing either (normal x86 behavior would be to drop writes and
> >> return all ones for reads).
> > 
> > How about doing the same we do for HVM which is inject GP. Then
> > handle_mmio would just return 0 for pvh, and
> > hvm_hap_nested_page_fault would not need to be modified.
> 
> The fundamentally wrong thing is that real hardware wouldn't
> surface #GP on any wrong physical address - with one exception,
> the only possibility would be #MC, and I don't think this would
> ever happen for truly unpopulated ranges. (The exception being
> on AMD, where #PF gets surfaced when trying to access a page
> referring to the HT reserved address range.) IOW even on HVM
> it is wrong for us to inject #GP in cases like this.

We, internally here, discussed the idea of injecting #PF instead of #GP. That 
makes sense from baremetal perspective. IOW, if we look at it from the 
prespective that the guest kernel put a bad pte entry, then on baremetal
it will get a #PF. Moreover, a PV guest returns 0 for mfn if the p2m
lookup fails, and in some paths it appears the code does do_mmu_update
subsequently.... which should result in guest getting a NULL ptr later. 

-Mukesh


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