kvm: Map PFN-type memory regions as writable (if possible)

For EPT-violations that are triggered by a read, the pages are also mapped with
write permissions (if their memory region is also writable). That would avoid
getting yet another fault on the same page when a write occurs.

This optimization only happens when you have a "struct page" backing the memory
region. So also enable it for memory regions that do not have a "struct page".

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

virt/kvm/kvm_main.c

@@ -1428,7 +1428,8 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
 
 static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       unsigned long addr, bool *async,
-			       bool write_fault, kvm_pfn_t *p_pfn)
+			       bool write_fault, bool *writable,
+			       kvm_pfn_t *p_pfn)
 {
 	unsigned long pfn;
 	int r;
@@ -1454,6 +1455,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 
 	}
 
+	if (writable)
+		*writable = true;
 
 	/*
 	 * Get a reference here because callers of *hva_to_pfn* and
@@ -1519,7 +1522,7 @@ retry:
 	if (vma == NULL)
 		pfn = KVM_PFN_ERR_FAULT;
 	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
-		r = hva_to_pfn_remapped(vma, addr, async, write_fault, &pfn);
+		r = hva_to_pfn_remapped(vma, addr, async, write_fault, writable, &pfn);
 		if (r == -EAGAIN)
 			goto retry;
 		if (r < 0)