linux-kernel - Re: [PATCH 1/3] kvm: wire up KVM_CAP_VM_GPA

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Date: Mon, 4 Mar 2024 12:47:22 +0100
From: Gerd Hoffmann <kraxel@...hat.com>
To: Tao Su <tao1.su@...ux.intel.com>
Cc: kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, 
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kvm: wire up KVM_CAP_VM_GPA_BITS for x86

On Mon, Mar 04, 2024 at 04:59:32PM +0800, Tao Su wrote:
> On Mon, Mar 04, 2024 at 09:43:53AM +0100, Gerd Hoffmann wrote:
> > > > +	kvm_caps.guest_phys_bits = boot_cpu_data.x86_phys_bits;
> > > 
> > > When KeyID_bits is non-zero, MAXPHYADDR != boot_cpu_data.x86_phys_bits
> > > here, you can check in detect_tme().
> > 
> > from detect_tme():
> > 
> >         /*
> >          * KeyID bits effectively lower the number of physical address
> >          * bits.  Update cpuinfo_x86::x86_phys_bits accordingly.
> >          */
> >         c->x86_phys_bits -= keyid_bits;
> > 
> > This looks like x86_phys_bits gets adjusted if needed.
> 
> If TDP is enabled and supports 5-level, we want kvm_caps.guest_phys_bits=52,
> but c->x86_phys_bits!=52 here.

Do you talk about EPT or NPT or both?

> Maybe we need to set kvm_caps.guest_phys_bits
> according to whether TDP is enabled or not, like leaf 0x80000008 in
> __do_cpuid_func().

See patches 2+3 of this series.

Maybe it is better to just not set kvm_caps.guest_phys_bits in generic
kvm code and leave that completely to vmx / svm vendor modules.  Or let
the generic code handle the !tdp_enabled case and have the vendor
modules override (considering EPT / NPT limitations) in case tdp is
enabled.

take care,
  Gerd