Re: [RFC PATCH 0/3] kvm/arm: New VMID allocator based on asid(2nd approach)

[Marc Zyngier]

On Fri, 04 Jun 2021 09:13:10 +0100,
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx> wrote:
> 
> Hi,
> 
> > -----Original Message-----
> > From: Shameerali Kolothum Thodi
> > Sent: 06 May 2021 17:52
> > To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx
> > Cc: maz@xxxxxxxxxx; will@xxxxxxxxxx; catalin.marinas@xxxxxxx;
> > james.morse@xxxxxxx; julien.thierry.kdev@xxxxxxxxx;
> > suzuki.poulose@xxxxxxx; jean-philippe@xxxxxxxxxx; Linuxarm
> > <linuxarm@xxxxxxxxxx>
> > Subject: [RFC PATCH 0/3] kvm/arm: New VMID allocator based on asid(2nd
> > approach)
> > 
> > This is based on a suggestion from Will [0] to try out the asid
> > based kvm vmid solution as a separate VMID allocator instead of
> > the shared lib approach attempted in v4[1].
> > 
> > The idea is to compare both the approaches and see whether the
> > shared lib solution with callbacks make sense or not.
> 
> A gentle ping on this. Please take a look and let me know.

I had a look and I don't overly dislike it. I'd like to see the code
without the pinned stuff though, at least to ease the reviewing. I
haven't tested it in anger, but I have pushed the rebased code at [1]
as it really didn't apply to 5.13-rc4.

One thing I'm a bit worried about is that we so far relied on VMID 0
never being allocated to a guest, which is now crucial for protected
KVM. I can't really convince myself that this can never happen with
this. Plus, I've found this nugget:

<quote
	max_pinned_vmids = NUM_USER_VMIDS - num_possible_cpus() - 2;
</quote>

What is this "- 2"? My hunch is that it should really be "- 1" as VMID
0 is reserved, and we have no equivalent of KPTI for S2.

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/mmu/vmid

-- 
Without deviation from the norm, progress is not possible.