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

[Wu, Feng]

> -----Original Message-----
> From: Radim Krčmář [mailto:rkrcmar@xxxxxxxxxx]
> Sent: Tuesday, November 24, 2015 10:32 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: pbonzini@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> 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)?

In my current implementation, I don't select a subset of vector's bits as
the number, instead, I use the whole vector number. For software emulation
p. o. v, do we really need to select a subset of the vector's bits as the base
number? What is your opinion? Thanks a lot!

Thank,
Feng
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