Re: [Qemu-devel] [PATCH v2] block: avoid SIGUSR2

[Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>]

On Fri, Oct 28, 2011 at 01:31:20PM +0100, Stefan Hajnoczi wrote:
>Subject: Re: [Qemu-devel] [PATCH v2] block: avoid SIGUSR2
>From: Stefan Hajnoczi <stefanha@gmail.com>
>To: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
>Cc: Paolo Bonzini <pbonzini@redhat.com>, Lucas Meneghel Rodrigues
> <lmr@redhat.com>, aliguori@us.ibm.com, Stefan Hajnoczi
> <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org, Frediano Ziglio
> <freddy77@gmail.com>, Cleber Rosa <crosa@redhat.com>, Kevin Wolf
> <kwolf@redhat.com>
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> UTC=2011-10-28 12:31:35
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>On Fri, Oct 28, 2011 at 1:29 PM, Kevin Wolf <kwolf@redhat.com> wrote:
>> Am 28.10.2011 13:50, schrieb Paolo Bonzini:
>>> On 10/28/2011 01:33 PM, Kevin Wolf wrote:
>>>> I'm afraid that we can only avoid things like this reliably if we
>>>> convert all devices to be direct users of AIO/coroutines. The current
>>>> block layer infrastructure doesn't emulate the behaviour of bdrv_read
>>>> accurately as bottom halves can be run in the nested main loop.
>>>>
>>>> For floppy, the following seems to be a quick fix (Lucas, Cleber, does
>>>> this solve your problems?), though it's not very satisfying. And I'm not
>>>> quite sure yet why it doesn't always happen with kill() in
>>>> posix-aio-compat.c.
>>>
>>> Another "fix" is to change idle bottom halves (at least the one in
>>> hw/dma.c) to 10ms timers.
>>
>> Which would be using the fact that timers are only executed in the real
>> main loop. Which makes me wonder if it would be enough for floppy if we
>> changed qemu_bh_poll() to take a bool run_idle_bhs that would be true in
>> the main loop and false an qemu_aio_wait().
>>
>> Still this wouldn't be a general solution as normal BHs have the very
>> same problem if they are scheduled before a bdrv_read/write call. To
>> solve that I guess we'd have to reintroduce AsyncContext, but it has its
>> own problems and was removed for a reason.
>>
>> Or we make some serious effort now to convert devices to AIO.
>
>Zhi Yong: We were just talking about converting devices to aio.  If
>you have time to do that for fdc, sd, or any other synchronous API
>users in hw/ that would be helpful.  Please let us know which device
>you are refactoring so we don't duplicate work.
Stefan,
I am working on flash(onenand, CFI), cdrom, sd, fdc, etc. If anyone has good thought, pls let me know.:)


Regards,

Zhi Yong Wu
>
>Stefan
>