Re: [Qemu-devel] [RFC PATCH 0/3] KVM: Introduce KVM_MEM_UNCACHED

[Paolo Bonzini]

On 18/03/2015 20:10, Andrew Jones wrote:
> Introduce a new memory region flag, KVM_MEM_UNCACHED, which
> is needed by ARM. This flag informs KVM that the given memory
> region is typically mapped by the guest as uncached. KVM for
> ARM then maps that region as uncached for userspace as well,
> in order to keep coherency.
> 
> Andrew Jones (3):
>   KVM: promote KVM_MEMSLOT_INCOHERENT to uapi
>   arm/arm64: KVM: decouple READONLY and UNCACHED
>   arm/arm64: KVM: implement KVM_MEM_UNCACHED
> 
>  Documentation/virtual/kvm/api.txt | 16 ++++---
>  arch/arm/include/asm/kvm_mmu.h    |  9 ++++
>  arch/arm/include/uapi/asm/kvm.h   |  2 +
>  arch/arm/kvm/arm.c                |  1 +
>  arch/arm/kvm/mmu.c                | 90 
> ++++++++++++++++++++++++++++++++++-----
>  arch/arm64/include/asm/kvm_mmu.h  |  9 ++++
>  arch/arm64/include/uapi/asm/kvm.h |  2 +
>  include/linux/kvm_host.h          |  1 -
>  include/uapi/linux/kvm.h          |  2 +
>  virt/kvm/kvm_main.c               |  7 ++-
>  10 files changed, 121 insertions(+), 18 deletions(-)
> 

I think the pinning breaks running KVM as non-root (the default ulimit
-l is just 64 KiB).  Can you do something about it with the MMU
notifiers instead?

It looks very clean.

The fly in the ointment is that some kind of quirk is probably needed in
the future for ivshmem---it's definitely not acceptable performance-wise
to mark it uncached, neither in the guests not in the host.  On the
other hand that quirk would be necessary only in the guest, not in the
firmware, so we can live with a mixture of the two approaches.

Paolo