fix memory access benchmark panicking on ARM

The benchmark tries to write to guest physical address 0, but on ARM
guest memory doesn't start at 0, so when resolving the address we
panicked.

Fix this by instead simply using whatever address guest memory starts
with.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/benches/memory_access.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use criterion::{criterion_group, criterion_main, BatchSize, Criterion};
-use vm_memory::{GuestAddress, GuestMemory};
+use vm_memory::GuestMemory;
 use vmm::resources::VmResources;
 use vmm::vmm_config::machine_config::{HugePageConfig, VmConfig};
 
@@ -11,11 +11,10 @@ fn bench_single_page_fault(c: &mut Criterion, configuration: VmResources) {
         b.iter_batched(
             || {
                 let memory = configuration.allocate_guest_memory().unwrap();
-                let ptr = memory
-                    .get_slice(GuestAddress(0), 1)
-                    .unwrap()
-                    .ptr_guard_mut()
-                    .as_ptr();
+                // Get a pointer to the first memory region (cannot do `.get_slice(GuestAddress(0),
+                // 1)`, because on ARM64 guest memory does not start at physical
+                // address 0).
+                let ptr = memory.iter().next().unwrap().as_ptr();
 
                 // fine to return both here, because ptr is not a reference into `memory` (e.g. no
                 // self-referential structs are happening here)