linux-kernel - Re: [PATCH v2] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

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Message-ID: <ZeGfQMo4TyTfw39y@liuwe-devbox-debian-v2>
Date: Fri, 1 Mar 2024 09:26:24 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Michael Kelley <mhklinux@...look.com>
Cc: Saurabh Sengar <ssengar@...ux.microsoft.com>,
	"kys@...rosoft.com" <kys@...rosoft.com>,
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>,
	"decui@...rosoft.com" <decui@...rosoft.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ssengar@...rosoft.com" <ssengar@...rosoft.com>
Subject: Re: [PATCH v2] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

On Tue, Jan 16, 2024 at 03:25:47PM +0000, Michael Kelley wrote:
> From: Saurabh Sengar <ssengar@...ux.microsoft.com> Sent: Monday, January 15, 2024 9:58 AM
> > 
> > The current method for signaling the compatibility of a Hyper-V host
> > with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
> > However, for higher VTLs, this leaf is not reported, due to the absence
> > of an IO-APIC.
> > 
> > As an alternative, assume that when running at a high VTL, the host
> > supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
> > employ any architectural MSIs at higher VTLs
> > 
> > This unblocks startup of VTL2 environments with more than 256 CPUs.
> > 
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
[...]
> 
> Reviewed-by: Michael Kelley <mhklinux@...look.com>

Applied. Thanks.