Hi Laszlo,
Thank you for your help.
On 26/10/17 16:20, Laszlo Ersek wrote:
On 10/26/17 17:13, Laszlo Ersek wrote:
Hello Julien,
On 10/26/17 13:05, Julien Grall wrote:
Hi all,
I was doing more testing of UEFI in Xen guests and noticed some slow
down when using the shell. The characters are only echoed after a second
or two that is a bit annoying.
The change that introduced this issue is 4cf3f37c87 "MdeModulePkg
SerialDxe: Process timeout consistently in SerialRead".
The Serial Driver for Xen PV console is very simple (see
OvmfPkg/Library/XenConsoleSerialPortLib). So I am not sure where the
root cause is.
Would anyone have any tips on it?
The exact same issue has been encountered earlier under QEMU, please
refer to the following sub-thread (please read it to end):
http://mid.mail-archive.com/b748580c-cb51-32c9-acf9-780841ef15da@redhat.com
The fix was commit 5f0f5e90ae8c ("ArmVirtPkg/FdtPL011SerialPortLib: call
PL011UartLib in all SerialPortLib APIs", 2017-08-16).
I think if you can implement the same for XenConsoleSerialPortLib, that
should return to working state as well.
Hmmm, wait, at a closer look, it looks like
OvmfPkg/Library/XenConsoleSerialPortLib/XenConsoleSerialPortLib.c
already implements that suggestion? (I.e., it sets
EFI_SERIAL_INPUT_BUFFER_EMPTY in *Control as necessary?)
Are we sure the SerialPortPoll() function works correctly? I don't see
any MemoryFence() calls in SerialPortPoll(), around checking the fields
in (*mXenConsoleInterface). Could that be the problem?
I am not entirely sure. But I added a couple of MemoryFence() in
SerialPort just in case to clear that from potential cause:
XENCONS_RING_IDX Consumer, Producer;
if (!mXenConsoleInterface) {
return FALSE;
}
MemoryFence ();
Consumer = mXenConsoleInterface->in_cons;
Producer = mXenConsoleInterface->in_prod;
MemoryFence ();
return Consumer != Producer;
I also added some debug printk (using a different interface) to confirm
the value of Consumer and Producer are valid. I can see the Producer
increasing every time a key is pressed and then soon followed by
SerialPortRead incrementing Consumer.
I did more debugging and find out the following is happening in
TerminalConInTimerHandler (MdeModulePkg/Universal/Console/TerminalDxe)
when a character is received:
1) GetControl will return EFI_SERIAL_INPUT_BUFFER_EMPTY unset
=> Entering in the loop to fetch character from the serial
2) GetOneKeyFromSerial()
=> Return directly with the character read
3) Looping as the fifo is not full and no error
4) GetOneKeyFromSerial() -> SerialRead()
=> No more character so SerialPortPoll() will return FALSE and loop
until timeout
=> Return EFI_TIMEOUT
5) Exiting the loop from TerminalConInTimerHandler
6) Characters are printed
So the step 4) will introduce the timeout seen and delay the echoing of
the characters received.
I could see a couple of solutions to fix it:
1) Remove the timeout from SerialPortRead and rely on either
a) caller to handle timeout
b) each UART driver
2) TerminalConInTimerHandler to check at every iteration whether the
buffer is empty.
Any other ideas?
Cheers,
--
Julien Grall
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