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/2018/3/7/621Signed-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>

arch/x86/xen/enlighten_pv.c

@@ -421,24 +421,15 @@ 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..
-	 */
-
-	BUG_ON(size > 65536);
-	BUG_ON(va & ~PAGE_MASK);
-
-	for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
+	unsigned long pfn, mfn;
 	int level;
 	pte_t *ptep;
-		unsigned long pfn, mfn;
 	void *virt;
 
+	/* @size should be at most GDT_SIZE which is smaller than PAGE_SIZE. */
+	BUG_ON(size > PAGE_SIZE);
+	BUG_ON(va & ~PAGE_MASK);
+
 	/*
 	 * The GDT is per-cpu and is in the percpu data area.
 	 * That can be virtually mapped, so we need to do a
@@ -453,13 +444,10 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
 	mfn = pfn_to_mfn(pfn);
 	virt = __va(PFN_PHYS(pfn));
 
-		frames[f] = mfn;
-
 	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();
 }
 
@@ -470,22 +458,13 @@ 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);
 
@@ -494,10 +473,7 @@ static void __init xen_load_gdt_boot(const struct desc_ptr *dtr)
 	if (HYPERVISOR_update_va_mapping((unsigned long)va, pte, 0))
 		BUG();
 
-		frames[f] = mfn;
-	}
-
-	if (HYPERVISOR_set_gdt(frames, size / sizeof(struct desc_struct)))
+	if (HYPERVISOR_set_gdt(&mfn, size / sizeof(struct desc_struct)))
 		BUG();
 }