linux-kernel - Re: [PATCH 3/4] KVM: selftests: Add VM_MODE_PXXV57

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Message-ID: <aPA-60vV0WQUCmc2@google.com>
Date: Wed, 15 Oct 2025 17:40:11 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>, 
	Andrew Jones <ajones@...tanamicro.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, 
	"Pratik R. Sampat" <prsampat@....com>, Kai Huang <kai.huang@...el.com>, 
	Eric Auger <eric.auger@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: selftests: Add VM_MODE_PXXV57_4K VM mode

On Wed, Oct 15, 2025, Sean Christopherson wrote:
> On Wed, Sep 17, 2025, Jim Mattson wrote:
> > Add a new VM mode, VM_MODE_PXXV57_4K, to support tests that require
> > 5-level paging on x86. This mode sets up a 57-bit virtual address
> > space and sets CR4.LA57 in the guest.

Thinking about this more, unless it's _really_ painful, e.g. because tests assume
4-level paging or 48-bit non-canonical address, I would rather turn VM_MODE_PXXV48_4K
into VM_MODE_PXXVXX_4K and have ____vm_create() create the "maximal" VM.  That
way tests don't need to go out of their way just to use 5-level paging, e.g. a
"TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_LA57))" is all that is needed.  It will also
gives quite a bit of coverage for free, e.g. that save/restore works with and
without 5-level paging (contrived example, but you get the point).

The NONCANONICAL #define works for LA57, so hopefully making tests play nice with
LA57 is straightforward?

> > @@ -358,6 +360,25 @@ struct kvm_vm *____vm_create(struct vm_shape shape)
> >  		vm->va_bits = 48;
> >  #else
> >  		TEST_FAIL("VM_MODE_PXXV48_4K not supported on non-x86 platforms");
> > +#endif
> > +		break;
> > +	case VM_MODE_PXXV57_4K:
> > +#ifdef __x86_64__
> > +		kvm_get_cpu_address_width(&vm->pa_bits, &vm->va_bits);
> > +		kvm_init_vm_address_properties(vm);
> > +		/*
> > +		 * For 5-level paging, KVM requires LA57 to be enabled, which
> > +		 * requires a 57-bit virtual address space.
> > +		 */
> > +		TEST_ASSERT(vm->va_bits == 57,
> > +			    "Linear address width (%d bits) not supported for VM_MODE_PXXV57_4K",
> > +			    vm->va_bits);
> > +		pr_debug("Guest physical address width detected: %d\n",
> > +			 vm->pa_bits);
> > +		vm->pgtable_levels = 5;
> > +		vm->va_bits = 57;
> > +#else
> > +		TEST_FAIL("VM_MODE_PXXV57_4K not supported on non-x86 platforms");
> >  #endif
> 
> That's a lot of copy+paste, especially given the #ifdefs.  How about this (untested)?
> 
> 	case VM_MODE_PXXV48_4K:
> 	case VM_MODE_PXXV57_4K:
> #ifdef __x86_64__
> 		kvm_get_cpu_address_width(&vm->pa_bits, &vm->va_bits);
> 		kvm_init_vm_address_properties(vm);
> 
> 		/*
> 		 * Ignore KVM support for 5-level paging (vm->va_bits == 57) if
> 		 * the target mode is 4-level paging (48-bit virtual address
> 		 * space), as 5-level paging only takes effect if CR4.LA57=1.
> 		 */
> 		TEST_ASSERT(vm->va_bits == 57 ||
> 			    (vm->va_bits == 48 && vm->mode == VM_MODE_PXXV48_4K),
> 			    "Linear address width (%d bits) not supported",
> 			    vm->va_bits);
> 		pr_debug("Guest physical address width detected: %d\n",
> 			 vm->pa_bits);
> 		if (vm->mode == VM_MODE_PXXV48_4K) {
> 			vm->pgtable_levels = 4;
> 			vm->va_bits = 48;
> 		} else {
> 			vm->pgtable_levels = 5;
> 			vm->va_bits = 57;
> 		}
> #else
> 		TEST_FAIL("VM_MODE_PXXV{48,57}_4K not supported on non-x86 platforms");
> #endif
> 		break;