x86/xen/p2m: hint at the last populated P2M entry

With commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (x86/xen: prepare
p2m list for memory hotplug) the P2M may be sized to accomdate a much
larger amount of memory than the domain currently has.

When saving a domain, the toolstack must scan all the P2M looking for
populated pages.  This results in a performance regression due to the
unnecessary scanning.

Instead of reporting (via shared_info) the maximum possible size of
the P2M, hint at the last PFN which might be populated.  This hint is
increased as new leaves are added to the P2M (in the expectation that
they will be used for populated entries).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: <stable@vger.kernel.org> # 4.0+
(cherry picked from commit 98dd166ea3a3c3b57919e20d9b0d1237fcd0349d)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit f80232d154a44edf89dc40a0754dd74628efce34)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 1f63ad2a9a49a94cf1a4f7ccf689cc48291012e2..788d68f29468832cab4c60fbd80c31d4b96375f3 100644 (file)
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -111,6 +111,15 @@ static unsigned long *p2m_identity;
 static pte_t *p2m_missing_pte;
 static pte_t *p2m_identity_pte;
 
+/*
+ * Hint at last populated PFN.
+ *
+ * Used to set HYPERVISOR_shared_info->arch.max_pfn so the toolstack
+ * can avoid scanning the whole P2M (which may be sized to account for
+ * hotplugged memory).
+ */
+static unsigned long xen_p2m_last_pfn;
+
 static inline unsigned p2m_top_index(unsigned long pfn)
 {
        BUG_ON(pfn >= MAX_P2M_PFN);
@@ -269,7 +278,7 @@ void xen_setup_mfn_list_list(void)
        else
                HYPERVISOR_shared_info->arch.pfn_to_mfn_frame_list_list =
                        virt_to_mfn(p2m_top_mfn);
-       HYPERVISOR_shared_info->arch.max_pfn = xen_max_p2m_pfn;
+       HYPERVISOR_shared_info->arch.max_pfn = xen_p2m_last_pfn;
        HYPERVISOR_shared_info->arch.p2m_generation = 0;
        HYPERVISOR_shared_info->arch.p2m_vaddr = (unsigned long)xen_p2m_addr;
        HYPERVISOR_shared_info->arch.p2m_cr3 =
@@ -405,6 +414,8 @@ void __init xen_vmalloc_p2m_tree(void)
        static struct vm_struct vm;
        unsigned long p2m_limit;
 
+       xen_p2m_last_pfn = xen_max_p2m_pfn;
+
        p2m_limit = (phys_addr_t)P2M_LIMIT * 1024 * 1024 * 1024 / PAGE_SIZE;
        vm.flags = VM_ALLOC;
        vm.size = ALIGN(sizeof(unsigned long) * max(xen_max_p2m_pfn, p2m_limit),
@@ -607,6 +618,12 @@ static bool alloc_p2m(unsigned long pfn)
                        free_p2m_page(p2m);
        }
 
+       /* Expanded the p2m? */
+       if (pfn > xen_p2m_last_pfn) {
+               xen_p2m_last_pfn = pfn;
+               HYPERVISOR_shared_info->arch.max_pfn = xen_p2m_last_pfn;
+       }
+
        return true;
 }