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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.04.16 at 15:25, <wei.liu2@xxxxxxxxxx> wrote:
> 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.

Yes. I've actually been working on this the last couple of hours.

> The patch itself
> is an improvement in its own right so it can go in:
> 
> Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

Thanks, Jan


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