Re: [Qemu-devel] KVM serial8250: too much work for irq3

[hpa]

On January 10, 2019 3:44:54 PM PST, Paolo Bonzini <address@hidden> wrote:
>On 22/09/17 15:28, Denys Zagorui wrote:
>> Hello
>> 
>> This is known issue, it was mentioned at first time about 9 years
>ago.
>> 
>> https://lkml.org/lkml/2008/2/5/401
>> 
>> https://bugzilla.redhat.com/show_bug.cgi?id=986761
>
>The issue is simply that QEMU disregards the baud rate that the guest
>programs into the serial port.  H. Peter Anvin's patch, which you
>mention, is only a band aid that is geared specifically at avoiding
>Linux's error.
>
>It was fixed for real in commit
>6936bfe514bbac7af4b24fad9ed9688b78b5be69
>by implementing a baud rate timer (May 2008), but then undone in
>fcfb4d6aae611d1f804d486d3c998000912c4c81 (March 2013) when the serial
>port got flow control support.
>
>Part of the reason may have been laziness, but also there are reasons
>to
>keep it this way.  Basically, the baud rate timer was implemented also
>in order to mitigate the problem with losing characters sent by the
>guest on the serial console; and thanks to flow control, QEMU will
>already slow down the transmit so that the other side never loses
>characters.  And because looking at logs on a serial console bps is
>slow
>and mildly annoying even at 115200 bps, it was decided that the "too
>much work" error was the lesser evil.
>
>Thanks,
>
>Paolo

In the case of the lesser evil it would seem that the hack I did would be good 
enough.

A hardware high bandwidth UART simulating device would have to do the best same 
in some way.
-- 
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