linux-kernel - Re: [Xen-devel] [PATCH v2 0/2] xen/pci: Use APIC for MSIs when APIC virtualization is supported

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Message-ID: <5450F2CE.8070104@citrix.com>
Date:	Wed, 29 Oct 2014 13:59:42 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<david.vrabel@...rix.com>, <konrad.wilk@...cle.com>,
	<stefano.stabellini@...citrix.com>
CC:	<linux-kernel@...r.kernel.org>, <jun.nakajima@...el.com>,
	<xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH v2 0/2] xen/pci: Use APIC for MSIs when APIC
 virtualization is supported

On 27/10/14 16:05, Boris Ostrovsky wrote:
> Changes in v2:
> * New version of cpuid.h file from Xen tree (with a couple of style adjustments)
> * Whitespace cleanup
> 
> Currently HVM guests handle MSI interrupts using pirqs/event channels, allowing
> us to not issue APIC accesses that result in somewhat expensive VMEXITs. When
> hardware supports APIC virtualization we don't need to use pirqs anymore
> since now guest's APIC accesses can be handled by the processor itself.
> 
> There are two patches in this series:
> 
> 1. Move setting of x86_msi ops to a later point. The reason for doing so is that
> we currently decide whether or not to use pirqs before kernel had a chance to
> see whether it should be using x2APIC instead of plain APIC. Since hardware may
> virtualize either or both of those two we can only make pirqs vs. APIC selection
> after kernel has settled down on which APIC version it will use. (Note that
> currently x2APIC is not used by HVM guests so technically this patch is not
> necessary. However, it probably makes sense to apply it now to avoid
> forgetting to do it when we enable x2APIC).
> 
> 2. Set x86_msi ops to use pirqs only when APIC virtualization is not available.
> The commit message describes performance improvements that this change brings.
> 
> Boris Ostrovsky (2):
>   xen/pci: Defer initialization of MSI ops on HVM guests until after
>     x2APIC has been set up
>   xen/pci: Use APIC directly when APIC virtualization is supported by
>     hardware

Applied to devel/for-linus-3.19.

Thanks.

David
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