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Re: [Xen-devel] Re: Next steps with pv_ops for Xen

To: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Next steps with pv_ops for Xen
From: Derek Murray <Derek.Murray@xxxxxxxxxxxx>
Date: Tue, 04 Dec 2007 09:40:49 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Eduardo Habkost <ehabkost@xxxxxxxxxx>, Juan Quintela <quintela@xxxxxxxxxx>, "Stephen C. Tweedie" <sct@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxxxx>, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, "virtualization@xxxxxxxxxxxxxx" <virtualization@xxxxxxxxxxxxxx>
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Gerd Hoffmann wrote:
On this point I completely agree with you! If anyone has any less
radical suggestions, then I'd be delighted to refactor the gntdev code
to use them. However, I'm not currently aware of any alternative that
maintains robustness to process crashes.

Oh, for me it isn't robust at all, it crashes on the first munmap
syscall.  It is the Fedora 8 kernel.  See attachment.  Didn't try
xensource 2.6.18 yet.

My gut feeling is that something changed in mm between 2.6.18 and 2.6.21, but that seems like a cop out so...

Ideas what is wrong?

Since the bug appears to be in page_remove_rmap, that would tend to imply that there is never a corresponding page_add_*_rmap (page_add_file_rmap?). My knowledge of the Linux mm code is a bit shaky here: should gntdev be doing this? Should we be using install_page (or a modified version thereof) to set the PTE?

Also, does a simple program that opens gntdev, maps a grant, accesses/writes to the page, and unmaps it (all using the xc_gnttab_* functions) work?

Who uses the gntdev device right now?

Good question! I'm aware of it being used in a few research projects, and it seems to work for them (though I think it is mostly used with the linux-2.6.18-xen kernel). Anyone else?

I think this would represent good progress, though I wonder if there
would be a performance penalty due to performing the mapping and
unmapping in user-space (multiple syscalls per mapping versus a single
hypercall).

I'd expect the hard disk (and how I/O is scheduled) being the
bottleneck, not the syscall overhead.  Nevertheless I plan to benchmark
it once I have it up and running.

Great to hear that you're working on this! Let me know if there's any other help I can provide with gntdev.

Cheers,

Derek.

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