linux-kernel - Re: [PATCH v3 1/2] kvm/cpuid: remove GuestPhysBits code.

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Date: Tue, 12 Mar 2024 10:38:04 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Gerd Hoffmann <kraxel@...hat.com>, kvm@...r.kernel.org
Cc: Tom Lendacky <thomas.lendacky@....com>,
 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 v3 1/2] kvm/cpuid: remove GuestPhysBits code.

On 3/11/2024 6:41 PM, Gerd Hoffmann wrote:
 > [PATCH v3 1/2] kvm/cpuid: remove GuestPhysBits code.

No need for a ending . in the title.

> GuestPhysBits (cpuid leaf 80000008, eax[23:16]) is intended for software
> use.  Physical CPUs do not set that field.  The current code which
> propagates the host's GuestPhysBits to the guest's PhysBits does not
> make sense.  Remove it.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> ---
>   arch/x86/kvm/cpuid.c | 32 ++++++++++++++------------------
>   1 file changed, 14 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index adba49afb5fe..c638b5fb2144 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1221,26 +1221,22 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   		entry->eax = entry->ebx = entry->ecx = 0;
>   		break;
>   	case 0x80000008: {
> -		unsigned g_phys_as = (entry->eax >> 16) & 0xff;
> -		unsigned virt_as = max((entry->eax >> 8) & 0xff, 48U);
> -		unsigned phys_as = entry->eax & 0xff;
> +		unsigned int virt_as = max((entry->eax >> 8) & 0xff, 48U);
> +		unsigned int phys_as;
>   
> -		/*
> -		 * If TDP (NPT) is disabled use the adjusted host MAXPHYADDR as
> -		 * the guest operates in the same PA space as the host, i.e.
> -		 * reductions in MAXPHYADDR for memory encryption affect shadow
> -		 * paging, too.
> -		 *
> -		 * If TDP is enabled but an explicit guest MAXPHYADDR is not
> -		 * provided, use the raw bare metal MAXPHYADDR as reductions to
> -		 * the HPAs do not affect GPAs.
> -		 */
> -		if (!tdp_enabled)
> -			g_phys_as = boot_cpu_data.x86_phys_bits;
> -		else if (!g_phys_as)
> -			g_phys_as = phys_as;
> +		if (!tdp_enabled) {
> +			/*
> +			 * If TDP (NPT) is disabled use the adjusted host
> +			 * MAXPHYADDR as the guest operates in the same PA
> +			 * space as the host, i.e.  reductions in MAXPHYADDR
> +			 * for memory encryption affect shadow paging, too.
> +			 */

I suggest keeping as the original comment to stay out of if check, and 
keeping the second paragraph with guest MAXPHYADDR thing removed as 
below, because the second paragraph is also useful.

   If TDP is enabled, use the raw bare metal MAXPHYADDR as reductions to
   the HPAs do not affect GPAs.

Otherwise,

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> +			phys_as = boot_cpu_data.x86_phys_bits;
> +		} else {
> +			phys_as = entry->eax & 0xff;
> +		}
>   
> -		entry->eax = g_phys_as | (virt_as << 8);
> +		entry->eax = phys_as | (virt_as << 8);
>   		entry->ecx &= ~(GENMASK(31, 16) | GENMASK(11, 8));
>   		entry->edx = 0;
>   		cpuid_entry_override(entry, CPUID_8000_0008_EBX);