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Re: [Xen-devel] [PATCH v4 0/2] xen/pci: Use APIC for MSIs when APIC virtualization is supported

On Tue, Dec 02, 2014 at 03:19:11PM -0500, Boris Ostrovsky wrote:
> Changes in v4:
> * Added comment describing what we check for in pci_xen_init()

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

> Changes in v3:
> * Explicitly include asm/apic.h in arch/x86/pci/xen.c for !CONFIG_SMP.
> 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
>  arch/x86/include/asm/xen/cpuid.h | 91 
> ++++++++++++++++++++++++++++++++++++++++
>  arch/x86/pci/xen.c               | 31 +++++++++++++-
>  2 files changed, 120 insertions(+), 2 deletions(-)
>  create mode 100644 arch/x86/include/asm/xen/cpuid.h
> -- 

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