Re: [PATCH v2 4/7] KVM: selftests: Correctly initialize the VA space for TTBR0_EL1

From: Oliver Upton <oliver.upton@linux.dev>
To: Sean Christopherson <seanjc@google.com>
Cc: Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	kvmarm@lists.linux.dev, Ricardo Koller <ricarkol@google.com>,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/7] KVM: selftests: Correctly initialize the VA space for TTBR0_EL1
Date: Fri, 9 Dec 2022 21:22:39 +0000	[thread overview]
Message-ID: <Y5OnH72knmPorYgn@google.com> (raw)
In-Reply-To: <Y5OeTeq55OgBJbjT@google.com>

On Fri, Dec 09, 2022 at 08:45:01PM +0000, Sean Christopherson wrote:
> On Fri, Dec 09, 2022, Oliver Upton wrote:
> > An interesting feature of the Arm architecture is that the stage-1 MMU
> > supports two distinct VA regions, controlled by TTBR{0,1}_EL1. As KVM
> > selftests on arm64 only uses TTBR0_EL1, the VA space is constrained to
> > [0, 2^(va_bits)). This is different from other architectures that
> > allow for addressing low and high regions of the VA space from a single
> > page table.
> > 
> > KVM selftests' VA space allocator presumes the valid address range is
> > split between low and high memory based the MSB, which of course is a
> > poor match for arm64's TTBR0 region.
> > 
> > Add a helper that correctly handles both addressing schemes with a
> > comment describing each.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> 
> Thanks much!  Looks awesome, especially the comment!
> 
> Reviewed-by: Sean Christopherson <seanjc@google.com>

ty!

> >  .../selftests/kvm/include/kvm_util_base.h     |  1 +
> >  tools/testing/selftests/kvm/lib/kvm_util.c    | 49 ++++++++++++++++---
> >  2 files changed, 44 insertions(+), 6 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
> > index 6cd86da698b3..b193863d754f 100644
> > --- a/tools/testing/selftests/kvm/include/kvm_util_base.h
> > +++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
> > @@ -103,6 +103,7 @@ struct kvm_vm {
> >  	struct sparsebit *vpages_mapped;
> >  	bool has_irqchip;
> >  	bool pgd_created;
> > +	bool has_split_va_space;
> >  	vm_paddr_t ucall_mmio_addr;
> >  	vm_paddr_t pgd;
> >  	vm_vaddr_t gdt;
> > diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> > index a256ec67aff6..53d15f32f220 100644
> > --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> > +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> > @@ -186,6 +186,43 @@ const struct vm_guest_mode_params vm_guest_mode_params[] = {
> >  _Static_assert(sizeof(vm_guest_mode_params)/sizeof(struct vm_guest_mode_params) == NUM_VM_MODES,
> >  	       "Missing new mode params?");
> >  
> > +/*
> > + * Initializes vm->vpages_valid to match the canonical VA space of the
> > + * architecture.
> > + *
> > + * Most architectures split the range addressed by a single page table into a
> > + * low and high region based on the MSB of the VA. On architectures with this
> > + * behavior the VA region spans [0, 2^(va_bits - 1)), [-(2^(va_bits - 1), -1].
> > + *
> > + * arm64 is a bit different from the rest of the crowd, as the low and high
> > + * regions of the VA space are addressed by distinct paging structures
> > + * (TTBR{0,1}_EL1).
> 
> Oooh, they're different CR3s in x86 terminology?

Right, we can have two active table roots at any given time, each
mapping their own portion of the address space.

--
Thanks,
Oliver

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