linux-kernel - Re: [PATCH 2/3] irqchip/gic-v3: allocate one SGI for MSHV

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Message-ID: <87bjkofpsc.wl-maz@kernel.org>
Date: Wed, 26 Nov 2025 21:27:15 +0000
From: Marc Zyngier <maz@...nel.org>
To: Anirudh Rayabharam <anirudh@...rudhrb.com>
Cc: kys@...rosoft.com,
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Subject: Re: [PATCH 2/3] irqchip/gic-v3: allocate one SGI for MSHV

On Wed, 26 Nov 2025 10:46:31 +0000,
Anirudh Rayabharam <anirudh@...rudhrb.com> wrote:
> 
> On Wed, Nov 26, 2025 at 09:02:30AM +0000, Marc Zyngier wrote:
> > On Wed, 26 Nov 2025 08:51:59 +0000,
> > Anirudh Rayabharam <anirudh@...rudhrb.com> wrote:
> > > 
> > > On Tue, Nov 25, 2025 at 06:01:38PM +0000, Marc Zyngier wrote:
> > > > On Tue, 25 Nov 2025 17:01:23 +0000,
> > > > Anirudh Raybharam <anirudh@...rudhrb.com> wrote:
> > > > > 
> > > > > From: Anirudh Rayabharam <anirudh@...rudhrb.com>
> > > > > 
> > > > > From: Anirudh Rayabharam (Microsoft) <anirudh@...rudhrb.com>
> > > > > 
> > > > > Currently SGIs are allocated only for the smp subsystem. The MSHV
> > > > > (Microsoft Hypervisor aka Hyper-V) code also needs an SGI that can be
> > > > > programmed into the SYNIC to receive intercepts from the hypervisor. The
> > > > > hypervisor would then assert this SGI whenever there is a guest
> > > > > VMEXIT.
> > > > > 
> > > > > Allocate one SGI for MSHV use in addition to the SGIs allocated for
> > > > > IPIs. When running under MSHV, the full SGI range can be used i.e. no
> > > > > need to reserve SGIs 8-15 for the secure firmware.
> > > > > 
> > > > > Since this SGI is needed only when running as a parent partition (i.e.
> > > > > we can create guest partitions), check for it before allocating an SGI.
> > > > 
> > > > Sorry, but that's not an acceptable situation.
> > > > 
> > > > SGIs are for Linux to use, nobody else, and that allocation must be
> > > 
> > > Why does this restriction exist? In the code SGIs 8-15 are left for
> > > secure firmware. So, things other than Linux can use SGIs. Why not MSHV
> > > then?
> > 
> > Because SGIs are for *internal* usage. Not usage from another random
> > piece of SW. The ACPI tables explicitly don't describe SGIs. DT
> > explicitly don't describe SGIs. Do you get the clue?
> 
> The name Software Generated Interrupts suggests that it is supposed to be
> used by pieces of SW.

I'm so glad you spell it out for me, I had no idea. I can't help but
notice that it is not called SGIFRPOSIDKA (Software Generated
Interrupt From Random Piece Of Software I Don't Know About).

Linux owns the SGIs, full stop. If you want to generate interrupts
from outside of Linux, use a standard interrupts. Not rocket science.

> Yes, ACPI/DT don't describe SGIs because they're not supposed to be used
> by devices. SW has full control over SGIs and it is up to the SW to
> assign meaning to them, isn't it?

No. It means that a single *consistent* software agent *owns* these
interrupts and doesn't have to let *anyone* else use them from outer
space.

> > > > the same irrespective of whether Linux runs virtualised or not. This
> > > > also won't work with GICv5 (there are no SGIs at all), so this is
> > > > doomed from the very start, and would immediately create technical
> > > > debt.
> > > 
> > > Hyper-V always presents a GICv3 so we don't need to worry about GICv5.
> > 
> > Well, that's pretty short sighted of you, and eventually you'll have
> > to support it, or just die. So do the right thing from the beginning.
> 
> Well, we don't when or if that will happen. But if it does happen, we
> can solve it in a way that makes sense for GICv5. If there are no SGIs
> at all, great, maybe we'll have a nicer solution then.

Great. See you then. In the meantime, I have no interest in this sort
of sorry hacks polluting the stuff I maintain.

> > > > If you want to signal an interrupt to Linux, expose a device with an
> > > > interrupt in a firmware table (i.e. not an SGI), and use that in your
> > > > driver.
> > > 
> > > You mean in the ACPI tables? That would require us to modify the
> > > firmware to expose this virtual device right?
> > 
> > Yes. How is that surprising?
> 
> It's not ideal that we would need some custom firmware to run Linux on
> MSHV (as a parent). Do you think there could be some other possible solution
> for handling this in the kernel? Maybe by thinking of it as a platform specific
> quirk or something?

You either do it the *correct* way, by exposing a virtual device, with
an edge-triggered PPI, just like other hypervisors have done, or you
keep your toy to yourself.  It is that simple. We don't have to accept
ugly crap in Linux just for the sake of you not having to do the right
thing in your firmware.

Feel free to post a new series once you have something that matches
the above expectations.

	M.

-- 
Jazz isn't dead. It just smells funny.