Re: [PATCH 4/4] arm/arm64: KVM: use kernel mapping to perform invalidation on page fault

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jan 13, 2015 at 01:35:16PM +0000, Christoffer Dall wrote:
> On Tue, Jan 13, 2015 at 12:12:41PM +0000, Peter Maydell wrote:
> > On 13 January 2015 at 12:04, Christoffer Dall
> > <christoffer.dall@linaro.org> wrote:
> > > Additionally, I haven't been able to think of a reasonable guest
> > > scenario where this breaks.  Once the guest turns on its MMU it should
> > > deal with the necessary icache invalidation itself (I think), so we're
> > > really talking about situations where the stage-1 MMU is off, and I
> > > gather that mostly you'll be seeing a single core doing any heavy
> > > lifting and then secondary cores basically coming up, only seeing valid
> > > entries in the icache, and doing the necessary invalidat+turn on mmu
> > > stuff.
> > 
> > The trouble with that is that as the secondary comes up, before it
> > turns on its icache its VA->PA mapping is the identity map; whereas
> > the primary vCPU's VA->PA mapping is "whatever the guest kernel's
> > usual mapping is". If the kernel has some mapping other than identity
> > for the VA which is wherever the secondary-CPU-startup-to-MMU-enable
> > code lives (which seems quite likely), then you have potential problems.
> > 
> Wouldn't a guest (and I believe Linux does this) reserve ASID 0 for
> additional cores and use ASID 1+++ for itself?

Not on arm since 52af9c6cd863fe37 (ARM: 6943/1: mm: use TTBR1 instead of
reserved context ID) and 45b95235b0ac86ce (ARM: 6944/1: mm: allow ASID 0
to be allocated to tasks). The swapper_pg_dir uses global mappings since
d427958a46af24f7 (ARM: 6942/1: mm: make TTBR1 always point to
swapper_pg_dir on ARMv6/7).

Similarly on arm64 the swapper_pg_dir and idmap_pg_dir use global
mappings and we don't reserve any ASIDs for use by the kernel.

Thanks,
Mark.