[Qemu-devel] Re: Write cache enable from guest at runtime

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Mar 15, 2011 at 10:50 AM, Christoph Hellwig <hch@lst.de> wrote:
> On Mon, Mar 14, 2011 at 07:15:14PM +0000, Stefan Hajnoczi wrote:
>> Sounds like a good idea.  Feel free to post the patches RFC and I or
>> someone else can debug and polish them if you don't have time.
>
> By looking at your document and doing what you recommend against I think I
> got a much simpler solution than what I had before.
>
> I didn't know we could actually write to the config space.  The vpcu
> synchronous behaviour is not a problem as it's done seldomly, and we
> can trivially check against errors by reading the value back.
>
> I've just started building this variant, and it seems surprisingly simple.

I didn't think about the just reading the value back.

What do you think about reopening the file via /proc/$pid/fd/$old_fd?
I wrote a test and had a look at the proc fd code.  It seems to work
fine and doesn't require an O_SYNC runtime change kernel patch.  It
allows us to fall back to the old fd if the new file cannot be opened
and it works even when the old file has been deleted.

Stefan