x86/xen: Remove use of VLAs

There's an ongoing effort to remove VLAs[1] from the kernel to eventually
turn on -Wvla. It turns out, the few VLAs in use in Xen produce only a
single entry array that is always bounded by GDT_SIZE. Clean up the code to
get rid of the VLA and the loop.

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

[boris: Use BUG_ON(size>PAGE_SIZE) instead of GDT_SIZE]
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

1 files changed, 31 insertions, 55 deletions

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index c36d23aa6c35..357969a3697c 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -421,45 +421,33 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
 {
         unsigned long va = dtr->address;
         unsigned int size = dtr->size + 1;
-        unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE);
-        unsigned long frames[pages];
-        int f;
-
-        /*
-         * A GDT can be up to 64k in size, which corresponds to 8192
-         * 8-byte entries, or 16 4k pages..
-         */
+        unsigned long pfn, mfn;
+        int level;
+        pte_t *ptep;
+        void *virt;
 
-        BUG_ON(size > 65536);
+        /* @size should be at most GDT_SIZE which is smaller than PAGE_SIZE. */
+        BUG_ON(size > PAGE_SIZE);
         BUG_ON(va & ~PAGE_MASK);
 
-        for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
-                int level;
-                pte_t *ptep;
-                unsigned long pfn, mfn;
-                void *virt;
-
-                /*
-                 * The GDT is per-cpu and is in the percpu data area.
-                 * That can be virtually mapped, so we need to do a
-                 * page-walk to get the underlying MFN for the
-                 * hypercall.  The page can also be in the kernel's
-                 * linear range, so we need to RO that mapping too.
-                 */
-                ptep = lookup_address(va, &level);
-                BUG_ON(ptep == NULL);
-
-                pfn = pte_pfn(*ptep);
-                mfn = pfn_to_mfn(pfn);
-                virt = __va(PFN_PHYS(pfn));
+        /*
+         * The GDT is per-cpu and is in the percpu data area.
+         * That can be virtually mapped, so we need to do a
+         * page-walk to get the underlying MFN for the
+         * hypercall.  The page can also be in the kernel's
+         * linear range, so we need to RO that mapping too.
+         */
+        ptep = lookup_address(va, &level);
+        BUG_ON(ptep == NULL);
 
-                frames[f] = mfn;
+        pfn = pte_pfn(*ptep);
+        mfn = pfn_to_mfn(pfn);
+        virt = __va(PFN_PHYS(pfn));
 
-                make_lowmem_page_readonly((void *)va);
-                make_lowmem_page_readonly(virt);
-        }
+        make_lowmem_page_readonly((void *)va);
+        make_lowmem_page_readonly(virt);
 
-        if (HYPERVISOR_set_gdt(frames, size / sizeof(struct desc_struct)))
+        if (HYPERVISOR_set_gdt(&mfn, size / sizeof(struct desc_struct)))
                 BUG();
 }
 
@@ -470,34 +458,22 @@ static void __init xen_load_gdt_boot(const struct desc_ptr *dtr)
 {
         unsigned long va = dtr->address;
         unsigned int size = dtr->size + 1;
-        unsigned pages = DIV_ROUND_UP(size, PAGE_SIZE);
-        unsigned long frames[pages];
-        int f;
-
-        /*
-         * A GDT can be up to 64k in size, which corresponds to 8192
-         * 8-byte entries, or 16 4k pages..
-         */
+        unsigned long pfn, mfn;
+        pte_t pte;
 
-        BUG_ON(size > 65536);
+        /* @size should be at most GDT_SIZE which is smaller than PAGE_SIZE. */
+        BUG_ON(size > PAGE_SIZE);
         BUG_ON(va & ~PAGE_MASK);
 
-        for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
-                pte_t pte;
-                unsigned long pfn, mfn;
+        pfn = virt_to_pfn(va);
+        mfn = pfn_to_mfn(pfn);
 
-                pfn = virt_to_pfn(va);
-                mfn = pfn_to_mfn(pfn);
+        pte = pfn_pte(pfn, PAGE_KERNEL_RO);
 
-                pte = pfn_pte(pfn, PAGE_KERNEL_RO);
-
-                if (HYPERVISOR_update_va_mapping((unsigned long)va, pte, 0))
-                        BUG();
-
-                frames[f] = mfn;
-        }
+        if (HYPERVISOR_update_va_mapping((unsigned long)va, pte, 0))
+                BUG();
 
-        if (HYPERVISOR_set_gdt(frames, size / sizeof(struct desc_struct)))
+        if (HYPERVISOR_set_gdt(&mfn, size / sizeof(struct desc_struct)))
                 BUG();
 }