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

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 08, 2022, Oliver Upton wrote:
> 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.

Ah, I missed the lack of a "-1" in the arm64 code.

> 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.

The main reason why I don't like splitting the code this way is that it makes it
harder for non-arm64 folks to understand what makes arm64 different.  Case in
point, my overlooking of the "-1".  I read the changelog and the comment and
still missed that small-but-important detail, largely because I am completely
unfamiliar with how TTBR{0,1}_EL1 works.

Actually, before we do anything, we should get confirmation from the s390 and
RISC-V folks on whether they have a canonical hole like x86, i.e. maybe x86 is
the oddball.

Anyways, assuming one architecture is the oddball (I'm betting it's x86), I have
no objection to bleeding some of the details into the common code, including a
large comment to document the gory details.  If every architecture manges to be
different, then yeah, a hook is probably warranted.

That said, I also don't mind shoving a bit of abstraction into arch code if that
avoids some #ifdef ugliness or allows for better documentation, flexibility, etc.
What I don't like is duplicating the logic of turning "VA bits" into the bitmap.

E.g. something like this would also be an option.  Readers would obviously need
to track down has_split_va_space, but that should be fairly easy and can come
with a big arch-specific comment, and meanwhile the core logic of how selftests
populate the va bitmaps is common.

Or if arm64 is the only arch without a split, invert the flag and have arm64 set
the vm->has_combined_va_space or whatever.

static void vm_vaddr_populate_bitmap(struct kvm_vm *vm)
{
	unsigned int eff_va_bits = vm->va_bits;
	sparsebit_num_t nr_bits;

	/* blah blah blah */
	if (vm->has_split_va_space)
		eff_va_bits--;

	nr_bits = (1ULL << eff_va_bits) >> vm->page_shift;

	sparsebit_set_num(vm->vpages_valid, 0, nr_va_bits);

	if (vm->has_split_va_space)
		sparsebit_set_num(vm->vpages_valid,
			  	  (~((1ULL << eff_va_bits) - 1)) >> vm->page_shift,
				  nr_bits);
}

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