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Re: [Xen-devel] Re: how to handle paged hypercall args?

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: how to handle paged hypercall args?
From: Keir Fraser <keir@xxxxxxx>
Date: Mon, 15 Nov 2010 10:33:44 +0000
Cc: Olaf Hering <olaf@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Patrick Colp <pjcolp@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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Thread-topic: [Xen-devel] Re: how to handle paged hypercall args?
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On 15/11/2010 10:20, "Tim Deegan" <Tim.Deegan@xxxxxxxxxx> wrote:

>> Yes, and you'd never turn on paging for dom0 itself. That would never work!
> 
> :) No, the issue is if dom0 (or whichever dom the pager lives in) is
> trying an operation on domU's memory that hits a paged-out page
> (e.g. qemu or similar is mapping it) with its only vpcu - you can't
> just block or spin.  You need to let dom0 schedule the pager process.
> 
>> Changing every user of the guest accessor macros to retry via guest space is
>> really not tenable. We'd never get all the bugs out.
> 
> Right now, I can't see another way of doing it.  Grants can be handled
> by shadowing the guest grant table and pinning granted frames so the
> block happens in domU (performance-- but you're already paging, right?)
> but what about qemu, xenctx, save/restore...?

We're talking about copy_to/from_guest, and friends, here. They always
implicitly act on the local domain, so the issue you raise is not a problem
there. Dom0 mappings of domU memory are a separate issue, presumably already
considered and dealt with to some extent, no doubt.

 -- Keir




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