Re: [PATCH 0/6] KVM: arm64: ... and FWB for all

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On Mon, Jan 19, 2026 at 10:56:45AM +0000, Marc Zyngier wrote:
> [Yes, you can tell what I was listening to]
> 
> Today, running in protected mode puts the host under it's own S2 with
> FWB=0, even if the rest of the guests are running with FWB=1. The
> rationale for this special-casing of the host is that we want the
> combined attributes to actually reflect the ones exposed by S1.
> 
> We therefore use NormalCacheable (which is the weakest memory type) as
> the default attributes at S2 so that S1 can only strengthen the final
> memory type.
> 
> But there is no reason why we cannot achieve the same effect with
> FWB. We normally use FWB to enforce cacheable memory from S2,
> irrespective of S1. But it is possible to configure the S2 attributes
> as "pass-through", so that the S1 attributes are always used.

Would you mind clarifying why FWB is preferable? Is it so that pkvm uses
the same approach for configuring stage 2 for the host as for a guest? Or
is it something else?

> 
> This small series implements that change, adding a flag that actually
> describes what we are trying to do (instead of the NOFWB flag which is
> pretty obscure), and fixes an interesting gotcha with CMOs.
> Interestingly, CMOs never actually worked with NOFWB on an FWB-aware
> platform...

Hmm... as far I can tell pkvm doesn't populate
mm_ops->dcache_clean_inval_poc for host_mmu, so pKVM doesn't issue CMOs for
the host s2, with or without these changes.

Thanks,
Alex

> 
> Patches on top of 6.19-rc5.
> 
> Marc Zyngier (6):
>   arm64: Add MT_S2{,_FWB}_AS_S1 encodings
>   KVM: arm64: Add KVM_PGTABLE_S2_AS_S1 flag
>   KVM: arm64: Make stage2_pte_cacheable() return false when S2_AS_S1 is
>     set
>   KVM: arm64: Switch pKVM host S2 over to KVM_PGTABLE_S2_AS_S1
>   KVM: arm64: Kill KVM_PGTABLE_S2_NOFWB
>   KVM: arm64: Simplify PAGE_S2_MEMATTR
> 
>  arch/arm64/include/asm/kvm_pgtable.h  |  7 +++---
>  arch/arm64/include/asm/memory.h       | 11 ++++++---
>  arch/arm64/include/asm/pgtable-prot.h |  4 ++--
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c |  4 +++-
>  arch/arm64/kvm/hyp/pgtable.c          | 32 +++++++++++++++++++--------
>  5 files changed, 39 insertions(+), 19 deletions(-)
> 
> -- 
> 2.47.3
> 
>