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

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Message-ID: <20240114092430.GA13684@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Sun, 14 Jan 2024 01:24:30 -0800
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: "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] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

On Wed, Jan 10, 2024 at 05:10:47AM +0000, Michael Kelley wrote:
> From: Saurabh Sengar <ssengar@...ux.microsoft.com> Sent: Friday, January 5, 2024 2:29 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 now deemed safe, as no
> > architectural MSIs are employed at higher VTLs.
> 
> I'm trying to fully understand this last sentence.  It has the words
> "now" and "deemed" as qualifiers.  Can you say anything more about
> why "now" (implying it wasn't safe at some point in the past)?
> And what are the implications of "deemed"?  Or are both just
> wordiness, and it would be just as good to say "This assumption is safe,
> as Hyper-V does not employ any architectural MSIs at higher VTLs." ?
> 
> The code LGTM.

Thank you for your review. Your phrasing appears better to me. I will revise
the commit message as per your suggestions and submit V2.

- Saurabh

> 
> Michael
> 
> > 
> > This unblocks startup of VTL2 environments with more than 256 CPUs.
> > 
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> > ---
> >  arch/x86/hyperv/hv_vtl.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> > index 539c7b5cfa2b..1c225362f88e 100644
> > --- a/arch/x86/hyperv/hv_vtl.c
> > +++ b/arch/x86/hyperv/hv_vtl.c
> > @@ -16,6 +16,11 @@
> >  extern struct boot_params boot_params;
> >  static struct real_mode_header hv_vtl_real_mode_header;
> > 
> > +static bool __init hv_vtl_msi_ext_dest_id(void)
> > +{
> > +	return true;
> > +}
> > +
> >  void __init hv_vtl_init_platform(void)
> >  {
> >  	pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
> > @@ -39,6 +44,8 @@ void __init hv_vtl_init_platform(void)
> >  	x86_platform.legacy.warm_reset = 0;
> >  	x86_platform.legacy.reserve_bios_regions = 0;
> >  	x86_platform.legacy.devices.pnpbios = 0;
> > +
> > +	x86_init.hyper.msi_ext_dest_id = hv_vtl_msi_ext_dest_id;
> >  }
> > 
> >  static inline u64 hv_vtl_system_desc_base(struct ldttss_desc *desc)
> > --
> > 2.25.1
> > 
>