xen/pvh: Don't setup P2M tree.

P2M is not available for PVH. Fortunatly for us the
P2M code already has mostly the support for auto-xlat guest thanks to
commit 3d24bbd7dddbea54358a9795abaf051b0f18973c
"grant-table: call set_phys_to_machine after mapping grant refs"
which: "
introduces set_phys_to_machine calls for auto_translated guests
(even on x86) in gnttab_map_refs and gnttab_unmap_refs.
translated by swiotlb-xen... " so we don't need to muck much.

with above mentioned "commit you'll get set_phys_to_machine calls
from gnttab_map_refs and gnttab_unmap_refs but PVH guests won't do
anything with them " (Stefano Stabellini) which is OK - we want
them to be NOPs.

This is because we assume that an "IOMMU is always present on the
plaform and Xen is going to make the appropriate IOMMU pagetable
changes in the hypercall implementation of GNTTABOP_map_grant_ref
and GNTTABOP_unmap_grant_ref, then eveything should be transparent
from PVH priviligied point of view and DMA transfers involving
foreign pages keep working with no issues[sp]

Otherwise we would need a P2M (and an M2P) for PVH priviligied to
track these foreign pages .. (see arch/arm/xen/p2m.c)."
(Stefano Stabellini).

We still have to inhibit the building of the P2M tree.
That had been done in the past by not calling
xen_build_dynamic_phys_to_machine (which setups the P2M tree
and gives us virtual address to access them). But we are missing
a check for xen_build_mfn_list_list - which was continuing to setup
the P2M tree and would blow up at trying to get the virtual
address of p2m_missing (which would have been setup by
xen_build_dynamic_phys_to_machine).

Hence a check is needed to not call xen_build_mfn_list_list when
running in auto-xlat mode.

Instead of replicating the check for auto-xlat in enlighten.c
do it in the p2m.c code. The reason is that the xen_build_mfn_list_list
is called also in xen_arch_post_suspend without any checks for
auto-xlat. So for PVH or PV with auto-xlat - we would needlessly
allocate space for an P2M tree.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

2 files changed, 11 insertions, 4 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index eb0efc2f9d3c..23ead298edbd 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1488,8 +1488,7 @@ asmlinkage void __init xen_start_kernel(void)
 	x86_configure_nx();
 
 	/* Get mfn list */
-	if (!xen_feature(XENFEAT_auto_translated_physmap))
-		xen_build_dynamic_phys_to_machine();
+	xen_build_dynamic_phys_to_machine();
 
 	/*
 	 * Set up kernel GDT and segment registers, mainly so that
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 2ae8699e8767..fb7ee0a4f9d0 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -280,6 +280,9 @@ void __ref xen_build_mfn_list_list(void)
 {
 	unsigned long pfn;
 
+	if (xen_feature(XENFEAT_auto_translated_physmap))
+		return;
+
 	/* Pre-initialize p2m_top_mfn to be completely missing */
 	if (p2m_top_mfn == NULL) {
 		p2m_mid_missing_mfn = extend_brk(PAGE_SIZE, PAGE_SIZE);
@@ -346,10 +349,15 @@ void xen_setup_mfn_list_list(void)
 /* Set up p2m_top to point to the domain-builder provided p2m pages */
 void __init xen_build_dynamic_phys_to_machine(void)
 {
-	unsigned long *mfn_list = (unsigned long *)xen_start_info->mfn_list;
-	unsigned long max_pfn = min(MAX_DOMAIN_PAGES, xen_start_info->nr_pages);
+	unsigned long *mfn_list;
+	unsigned long max_pfn;
 	unsigned long pfn;
 
+	 if (xen_feature(XENFEAT_auto_translated_physmap))
+		return;
+
+	mfn_list = (unsigned long *)xen_start_info->mfn_list;
+	max_pfn = min(MAX_DOMAIN_PAGES, xen_start_info->nr_pages);
 	xen_max_p2m_pfn = max_pfn;
 
 	p2m_missing = extend_brk(PAGE_SIZE, PAGE_SIZE);