linux-kernel - Re: [PATCH v8 29/33] x86/fred: allow FRED systems to use interrupt vectors 0x10-0x1f

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Date:   Mon, 5 Jun 2023 09:55:40 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>, Xin Li <xin3.li@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        peterz@...radead.org, andrew.cooper3@...rix.com, seanjc@...gle.com,
        pbonzini@...hat.com, ravi.v.shankar@...el.com,
        jiangshanlai@...il.com, shan.kang@...el.com
Subject: Re: [PATCH v8 29/33] x86/fred: allow FRED systems to use interrupt
 vectors 0x10-0x1f

On 6/5/23 07:06, Thomas Gleixner wrote:
> On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>>
>> FRED inherits the Intel VT-x enhancement of classified events with
>> a two-level event dispatch logic. The first-level dispatch is on
>> the event type, and the second-level is on the event vector. This
>> also means that vectors in different event types are orthogonal,
>> thus, vectors 0x10-0x1f become available as hardware interrupts.
>>
>> Enable interrupt vectors 0x10-0x1f on FRED systems (interrupt 0x80 is
>> already enabled.) Most of these changes are about removing the
>> assumption that the lowest-priority vector is hard-wired to 0x20.
> 
> I'm not really interested in this again premature optimization.
> 
> Can we please clarify how the final result of FRED vector layout will
> look like?
> 
> I rather give up on reclaiming these 16 vectors than making _all_ system
> vectors dynamically assignable to avoid an extra partitioning of the
> vector space.
> 

So this patch was meant to go *after* the FRED patch set proper.

This patch does not change any of the system vectors except, by 
necessity, IRQ_MOVE_CLEANUP_VECTOR, nor does it change the way they are 
assigned to fixed vectors today.

It does make the lowest *non*-system vector a variable, but the system 
vectors are at the end.

This is something that should be commented: the setting of bits in 
system_vector is really misleading: those aren't "system vectors", they 
are the vectors reserved for hardware use. On IDT, this means vectors 
0-31 because they are used for exceptions, but on FRED interrupts and 
exceptions are separate; however vectors 0-15 still need to be reserved 
because the APIC doesn't support them.

We *could* change that, but that is completely independent of this.

	-hpa