Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

[Ard Biesheuvel]

On 20 November 2017 at 18:06, Will Deacon <will.deacon@xxxxxxx> wrote:
> Hi Ard,
>
> Cheers for having a look.
>
> On Sat, Nov 18, 2017 at 03:25:06PM +0000, Ard Biesheuvel wrote:
>> On 17 November 2017 at 18:21, Will Deacon <will.deacon@xxxxxxx> wrote:
>> > This patch series implements something along the lines of KAISER for arm64:
>>
>> Very nice! I am quite pleased, because this makes KASLR much more
>> useful than it is now.
>
> Agreed. I might actually start enabling now ;)
>

I think it makes more sense to have enabled on your phone than on the
devboard on your desk.

>> My main question is why we need a separate trampoline vector table: it
>> seems to me that with some minor surgery (as proposed below), we can
>> make the kernel_ventry macro instantiations tolerant for being loaded
>> somewhere in the fixmap (which I think is a better place for this than
>> at the base of the VMALLOC space), removing the need to change
>> vbar_el1 back and forth. The only downside is that exceptions taken
>> from EL1 will also use absolute addressing, but I don't think that is
>> a huge price to pay.
>
> I think there are two aspects to this:
>
> 1. Moving the vectors to the fixmap
> 2. Avoiding the vbar toggle
>
> I think (1) is a good idea, so I'll hack that up for v2. The vbar toggle
> isn't as obvious: avoiding it adds some overhead to EL1 irq entry because
> we're writing tpidrro_el0 as well as loading from the literal pool.

Yeah, but in what workloads are interrupts taken while running in the
kernel a dominant factor?

> I think
> that it also makes the code more difficult to reason about because we'd have
> to make sure we don't try to use the fixmap mapping before it's actually
> mapped, which I think would mean we'd need a set of early vectors that we
> then switch away from in a CPU hotplug notifier or something.
>

I don't think this is necessary. The vector page with absolute
addressing would tolerate being accessed via its natural mapping
inside the kernel image as well as via the mapping in the fixmap
region.

> I'll see if I can measure the cost of the current vbar switching to get
> an idea of the potential performance available.
>

Yeah, makes sense. If the bulk of the performance hit is elsewhere,
there's no point in focusing on this bit.