linux-kernel - Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC version 0x20 with EOIR

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Message-ID: <9a04f3dda43aa50e2a160ccfd57d0d4f168b3dce.camel@infradead.org>
Date: Mon, 29 Dec 2025 13:01:07 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Khushit Shah <khushit.shah@...anix.com>
Cc: "seanjc@...gle.com" <seanjc@...gle.com>, "pbonzini@...hat.com"
 <pbonzini@...hat.com>, "kai.huang@...el.com" <kai.huang@...el.com>, 
 "mingo@...hat.com" <mingo@...hat.com>, "x86@...nel.org" <x86@...nel.org>,
 "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,  "dave.hansen@...ux.intel.com"
 <dave.hansen@...ux.intel.com>, "tglx@...utronix.de" <tglx@...utronix.de>, 
 Jon Kohler <jon@...anix.com>, Shaju Abraham <shaju.abraham@...anix.com>
Subject: Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC
 version 0x20 with EOIR

On Mon, 2025-12-29 at 12:21 +0000, Khushit Shah wrote:
> 
> > On 29 Dec 2025, at 5:09 PM, David Woodhouse <dwmw2@...radead.org>
> > wrote:
> > 
> > On 29 December 2025 11:17:07 GMT, Khushit Shah
> > <khushit.shah@...anix.com> wrote:
> > > From: David Woodhouse <dwmw@...zon.co.uk>
> > > 
> > > Introduce support for I/O APIC version 0x20, which includes the
> > > EOI
> > > Register (EOIR) for directed EOI.  The EOI register allows guests
> > > to
> > > perform EOIs to individual I/O APICs instead of relying on
> > > broadcast EOIs
> > > from the local APIC.
> > > 
> > > When Suppress EOI Broadcast (SEOIB) capability is advertised to
> > > the guest,
> > > guests that enable it will EOI individual I/O APICs by writing to
> > > their
> > > EOI register instead of relying on broadcast EOIs from the
> > > LAPIC.  Hence,
> > > when SEOIB is advertised (so that guests can use it if they
> > > choose), use
> > > I/O APIC version 0x20 to provide the EOI register.  This prepares
> > > for a
> > > userspace API that will allow explicit control of SEOIB support,
> > > providing
> > > a consistent interface for both in-kernel and split IRQCHIP mode.
> > > 
> > > Add a tracepoint (kvm_ioapic_directed_eoi) to track directed EOIs
> > > for
> > > debugging and observability.
> > > 
> > > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > > Signed-off-by: Khushit Shah <khushit.shah@...anix.com>
> > > ---
> > > arch/x86/kvm/ioapic.c | 31 +++++++++++++++++++++++++++++--
> > > arch/x86/kvm/ioapic.h | 19 +++++++++++--------
> > > arch/x86/kvm/trace.h  | 17 +++++++++++++++++
> > > 3 files changed, 57 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> > > index 6bf8d110aece..eea1eb7845c4 100644
> > > --- a/arch/x86/kvm/ioapic.c
> > > +++ b/arch/x86/kvm/ioapic.c
> > > @@ -48,8 +48,11 @@ static unsigned long
> > > ioapic_read_indirect(struct kvm_ioapic *ioapic)
> > > 
> > > switch (ioapic->ioregsel) {
> > > case IOAPIC_REG_VERSION:
> > > - result = ((((IOAPIC_NUM_PINS - 1) & 0xff) << 16)
> > > -   | (IOAPIC_VERSION_ID & 0xff));
> > > + if (kvm_lapic_advertise_suppress_eoi_broadcast(ioapic->kvm))
> > > + result = IOAPIC_VERSION_ID_EOIR;
> > > + else
> > > + result = IOAPIC_VERSION_ID;
> > > + result |= ((IOAPIC_NUM_PINS - 1) & 0xff) << 16;
> > 
> > I think that wants to depend on _respect_ not _advertise_?
> > Otherwise you're changing existing behaviour in the legacy/quirk
> > case where the VMM neither explicitly enables not disables the
> > feature.
> 
> I think _advertise_ is correct, as for legacy case, in kernel IRQCHIP
> mode, _advertise_ is false. For kernel IRQCHIP, _advertise_ is only
> true when *enabled*.

Hm? IIUC kvm_lapic_advertise_suppress_eoi_broadcast() is true whenever
userspace *hasn't* set KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST
(either userspace has explicitly *enabled* it instead, or userspace has
done neither and we should preserve the legacy behaviour).

If the kernel I/O APIC is enabled when userspace has not explicitly
either enabled or disabled EOI suppression, then the I/O APIC should
advertise precisely the same features as before. As far as I can tell,
this will make the kernel I/O APIC advertise the newer version and
support the EOI register in that legacy case, which it shouldn't?


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