crosvm: make port 0x61 return 0x20, preventing Guest kernel hang

Harmonize small 8042 implementation with kvmtool by making read from
port 0x61 return 0x20. This prevents infinite loop in
pit_calibrate_tsc() on small Intel cores.

BUG=none
TEST=reproduced problem on Celeraon N3150 (Braswell), verified
     VM boots further with the fix

Change-Id: Ifbee2bc72c4f5acd2551d8dcab466dc5f4907975
Reviewed-on: https://chromium-review.googlesource.com/938759
Commit-Ready: Slava Malyugin <slavamn@chromium.org>
Tested-by: Slava Malyugin <slavamn@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/devices/src/i8042.rs b/devices/src/i8042.rs
index b046184..c23dd03 100644
--- a/devices/src/i8042.rs
+++ b/devices/src/i8042.rs
@@ -18,10 +18,17 @@ impl I8042Device {
     }
 }
 
+// i8042 device is mapped I/O address 0x61. We partially implement two 8-bit
+// registers: port 0x61 (I8042_PORT_B_REG, offset 0 from base of 0x61), and
+// port 0x64 (I8042_COMMAND_REG, offset 3 from base of 0x61).
 impl BusDevice for I8042Device {
     fn read(&mut self, offset: u64, data: &mut [u8]) {
         if data.len() == 1 && offset == 3 {
             data[0] = 0x0;
+        } else if data.len() == 1 && offset == 0 {
+            // Like kvmtool, we return bit 5 set in I8042_PORT_B_REG to
+            // avoid hang in pit_calibrate_tsc() in Linux kernel.
+            data[0] = 0x20;
         }
     }