KVM: SVM: Fix offset computation bug in __sev_dbg_decrypt().

Fix offset computation in __sev_dbg_decrypt() to include the
source paddr before it is rounded down to be aligned to 16 bytes
as required by SEV API. This fixes incorrect guest memory dumps
observed when using qemu monitor.

Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Message-Id: <20201110224205.29444-1-Ashish.Kalra@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -642,8 +642,8 @@ static int __sev_dbg_decrypt(struct kvm *kvm, unsigned long src_paddr,
 	 * Its safe to read more than we are asked, caller should ensure that
 	 * destination has enough space.
 	 */
-	src_paddr = round_down(src_paddr, 16);
 	offset = src_paddr & 15;
+	src_paddr = round_down(src_paddr, 16);
 	sz = round_up(sz + offset, 16);
 
 	return __sev_issue_dbg_cmd(kvm, src_paddr, dst_paddr, sz, err, false);