Re: [PATCH 03/21] Remove use of signalfd in block-raw-posix.c

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> We don't use signalfd in upstream QEMU.  Instead, we always emulate it.
>>   
>
> With an extra thread -> so an extra context switch.

We don't use an extra thread.  We just install a signal handler that 
writes to a pipe.  At best, the added overhead is that we get EINTRs 
more often but this is something we already handle.

>> It's not necessarily a bad thing to use signalfd, but this is 
>> something that
>> should be done upstream.  It certainly does qemu-kvm no harm to use 
>> the upstream
>> code.
>>   
>
> It will introduce a (likely minor, but real) performance regression.
>
> Instead of this, why not apply the reverse patch to qemu.git?

I'm not sure signalfd really buys us much.  To emulate it requires 
writing a bunch more data to the pipe.  When writing more than 1 byte, 
we have to worry about whether there's a partial write because the pipe 
buffers full).  We also have to make sure to read from the fd in 
properly sized chunks.

Regards,

Anthony Liguori