| /* |
| * Copyright 2019 Google LLC |
| * |
| * Licensed under both the 3-Clause BSD License and the GPLv2, found in the |
| * LICENSE and LICENSE.GPL-2.0 files, respectively, in the root directory. |
| * |
| * SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0 |
| */ |
| |
| .global MeasureReadLatency |
| // uint64_t MeasureReadLatency(const void* address); |
| MeasureReadLatency: |
| // x0 = address |
| |
| // Full memory and speculation barrier. See docs/fencing.md for details. |
| // |
| // Of special note: we need the ISB here to prevent some processors from |
| // speculating ahead and reading the timestamp counter early. DSB doesn't |
| // stop later instructions from "[r]eading ... System registers that are |
| // directly or indirectly read without causing side-effects"[1], which seems |
| // to include the virtual count. |
| // |
| // Linux adds an ISB before reading CNTVCT_EL0: https://git.io/Jeivz |
| // |
| // [1] DSB: https://cpu.fyi/d/047#G9.10258412 |
| dsb sy |
| isb |
| |
| // x1 = <virtual count> |
| // CNTVCT_EL0: https://cpu.fyi/d/047#G31.5432229 |
| mrs x1, cntvct_el0 |
| |
| // Finish reading the virtual count before starting the read. |
| dsb sy |
| |
| // Read *address. |
| ldrb w0, [x0] |
| |
| // Finish the read before reading the virtual count again. As before, we need |
| // ISB to prevent the timestamp read from issuing early. |
| dsb sy |
| isb |
| |
| // x2 = <virtual count> |
| mrs x2, cntvct_el0 |
| |
| // x0 = x2 - x1 |
| sub x0, x2, x1 |
| |
| ret |
