Re: Xen Console input very slow in recent UEFI

[Laszlo Ersek <lersek@redhat.com>]

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?

Thanks,
Laszlo


> 
> ... This is why we need active Xen participants in edk2 ;)
> 
> Thanks!
> Laszlo
>