linux-kernel - Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Date:	Tue, 24 Nov 2015 15:31:55 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>
Cc:	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts

2015-11-24 01:26+0000, Wu, Feng:
> "I don't think we do any vector hashing on our client parts.  This may be why the customer is not able to detect this on Skylake client silicon.
> The vector hashing is micro-architectural and something we had done on server parts.
> 
> If you look at the haswell server CPU spec (https://www-ssl.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e5-v3-datasheet-vol-2.pdf)
> In section 4.1.2, you will see an IntControl register (this is a register controlled/configured by BIOS) - see below.

Thank you!

> If you look at bits 6:4 in that register, you see the option we offer in hardware for what kind of redirection is applied to lowest priority interrupts.
> There are three options:
> 1.	Fixed priority  
> 2.	Redirect last 
> 3.	Hash Vector
> 
> If picking vector hash, then bits 10:8 specifies the APIC-ID bits used for the hashing."

The hash function just interprets a subset of vector's bits as a number
and uses that as a starting offset in a search for an enabled APIC
within the destination set?

For example:
The x2APIC destination is 0x00000055 (= first four even APICs in cluster
0), the vector is 0b11100000, and bits 10:8 of IntControl are 000.

000 means that bits 7:4 of vector are selected, thus the vector hash is
0b1110 = 14, so the round-robin effectively does 14 % 4 (because we only
have 4 destinations) and delivers to the 3rd possible APIC (= ID 6)?
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