Re: [Xen-devel] [PATCH] x86/vMSI-X: avoid missing first unmask of vectors

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Apr 21, 2016 at 03:38:26AM -0600, Jan Beulich wrote:
> Recent changes to Linux result in there just being a single unmask
> operation prior to expecting the first interrupts to arrive. However,
> we've had a chicken-and-egg problem here: Qemu invokes
> xc_domain_update_msi_irq(), ultimately leading to
> msixtbl_pt_register(), upon seeing that first unmask operation. Yet
> for msixtbl_range() to return true (in order to msixtbl_write() to get
> invoked at all) msixtbl_pt_register() must have completed.
> 
> Deal with this by snooping suitable writes in msixtbl_range() and
> triggering the invocation of msix_write_completion() from
> msixtbl_pt_register() when that happens in the context of a still in
> progress vector control field write.
> 
> Note that the seemingly unrelated deletion of the redundant
> irq_desc->msi_desc checks in msixtbl_pt_register() is to make clear to
> any compiler version used that the "msi_desc" local variable isn't
> being used uninitialized. (Doing the same in msixtbl_pt_unregister() is
> just for consistency reasons.)
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> TODO: How to deal with REP MOVS to the MSI-X table (in msixtbl_range())?
> 

As I understand it, this should be future improvement. The patch itself
is an improvement in its own right so it can go in:

Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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