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Re: [Xen-devel] [PATCH 0/3] xen: msi support for Xen dom0

To: Qing He <qing.he@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 0/3] xen: msi support for Xen dom0
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 18 Aug 2009 13:24:26 -0700
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On 08/17/09 22:45, Qing He wrote:
> This patch set adds support for msi in Xen dom0. It's based on the
> pci notifier patches of Weidong Han (on rebase/pci branch) and
> contains the following 3 patches.
>       [PATCH 1/3] xen: make pci notifier work with booting devices
>       [PATCH 2/3] xen: add msi support for dom0
>       [PATCH 3/3] xen: re-enable msi (effectively revert bf89bc29)

Thanks, I've applied these to rebase/dom0/msi for now.   I haven't
tested them (or really compiled them) yet, so please look at the branch
and see that everything's OK.

> One of the problem left is how to save/restore MSI across S3. Since
> pci_restore_msi_state() now doesn't have any arch specific hook, the
> code in arch/x86/ won't get a chance to run during S3 wakeup, so
> write_msi_msg() is called instead of xen specific functions. One of
> the possible solutions (and which I prefer) is to add something like
> arch_pci_restore_msi, but that involves slightly changing
> drivers/pci/msi.c, which probably needs more thinking and discussion.
> An alternative is to trap and emulate any access to pci configuration
> space. In that case, nothing in dom0 needs changing, and write_msi_msg
> can be reused, but considerable logic may need to change in Xen
> hypervisor.

The approach taken by 2/3 is not really going to fly upstream, and is
broadly incompatible with my intended design for interrupt handling,
which is to decouple the Xen/dom0 aspects of interrupt handling from the
apic/ioapic code entirely.  I don't know what impact this will have on
MSI support.  I'd appreciate it if you could look at the
rebase/dom0/new-interrupt-routing branch and comment on it.

I'm not actually sure this approach is going to work; so far it just
locks up the machine shortly after ACPI initialization.  Trapping an
emulating (IO-)APIC accesses may well turn out to be simpler (on the
Linux side, at least) and more robust in the end...


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