linux-kernel - Re: [PATCH 5/5] x86/Hyper-V: Add Hyper-V specific hvcall to set backing page

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Message-ID: <CAMvTesBwkFr7VYoEYq5hc7NJi5xdiz037bmzoaEV2eG__h-kTg@mail.gmail.com>
Date: Fri, 19 Sep 2025 01:11:02 +0800
From: Tianyu Lan <ltykernel@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org, 
	decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, arnd@...db.de, 
	Neeraj.Upadhyay@....com, tiala@...rosoft.com, kvijayab@....com, 
	romank@...ux.microsoft.com, linux-arch@...r.kernel.org, 
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Michael Kelley <mhklinux@...look.com>
Subject: Re: [PATCH 5/5] x86/Hyper-V: Add Hyper-V specific hvcall to set
 backing page

On Thu, Sep 18, 2025 at 11:10 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Sep 18, 2025 at 11:00:23AM -0400, Tianyu Lan wrote:
> > Secure AVIC hardware provides APIC backing page
> > to aid the guest in limiting which interrupt
> > vectors can be injected into the guest. Hyper-V
> > introduces a new register HV_X64_REGISTER_SEV_GPA_PAGE
> > to notify hypervisor with APIC backing page and call
> > it in Secure AVIC driver.
>
> Why does hyperv needs special handling again and cannot simply adhere to the
> secure AVIC spec?
>
> None of that text explains *why* it is absolutely necessary to do something
> hyperv-special...

Hyper-V uses a different hvcall to register an APIC backing page.

>
> > @@ -361,7 +364,11 @@ static void savic_setup(void)
> >        * VMRUN, the hypervisor makes use of this information to make sure
> >        * the APIC backing page is mapped in NPT.
> >        */
> > -     res = savic_register_gpa(gpa);
> > +     if (hv_isolation_type_snp())
> > +             res = hv_set_savic_backing_page(gfn);
> > +     else
> > +             res = savic_register_gpa(gpa);
> > +
>
> This is ugly and doesn't belong here.
>

Could I move the check into savic_register_gpa() or add a stub function
to check guest runs on Hyper-V  or not and then call associated function
to register APIC backing page?

--
Thanks

Tianyu Lan