linux-kernel - Re: [PATCH 3/4] KVM: arm64: selftests: Align VA space allocator with TTBR0

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Date:   Thu, 8 Dec 2022 00:27:02 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Ricardo Koller <ricarkol@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: arm64: selftests: Align VA space allocator with
 TTBR0

On Thu, Dec 08, 2022 at 12:18:07AM +0000, Sean Christopherson wrote:

[...]

> Together, what about?  The #ifdef is a bit gross, especially around "hi_start",
> but it's less duplicate code.  And IMO, having things bundled in the same place
> makes it a lot easier for newbies (to arm64 or kernel coding in general) to
> understand what's going on and why arm64 is different.

I'd rather we not go this route. We really shouldn't make any attempt to
de-dupe something that is inherently architecture specific.

For example:

> +	/*
> +	 * All architectures supports splitting the virtual address space into
> +	 * a high and a low half.  Populate both halves, except for arm64 which
> +	 * currently uses only TTBR0_EL1 (arbitrary selftests "logic"), i.e.
> +	 * only has a valid low half.
> +	 */
> +	sparsebit_num_t nr_va_bits = (1ULL << (vm->va_bits - 1)) >> vm->page_shift;

This is still wrong for arm64. When we say the VA space is 48 bits, we
really do mean that TTBR0 is able to address a full 48 bits. So this
truncates the MSB for the addressing mode.

With the code living in the arm64 side of the shop, I can also tailor
the comment to directly match the architecture to provide breadcrumbs
tying it back to the Arm ARM.

--
Thanks,
Oliver