Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM

From: Suzuki K Poulose
Date: Wed Jul 04 2018 - 18:03:24 EST

On 07/04/2018 04:51 PM, Will Deacon wrote:
Hi Suzuki,

On Fri, Jun 29, 2018 at 12:15:35PM +0100, Suzuki K Poulose wrote:
Allow specifying the physical address size for a new VM via
the kvm_type argument for KVM_CREATE_VM ioctl. This allows
us to finalise the stage2 page table format as early as possible
and hence perform the right checks on the memory slots without
complication. The size is encoded as Log2(PA_Size) in the bits[7:0]
of the type field and can encode more information in the future if
required. The IPA size is still capped at 40bits.

Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
Cc: Christoffer Dall <cdall@xxxxxxxxxx>
Cc: Peter Maydel <peter.maydell@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
arch/arm/include/asm/kvm_mmu.h | 2 ++
arch/arm64/include/asm/kvm_arm.h | 10 +++-------
arch/arm64/include/asm/kvm_mmu.h | 2 ++
include/uapi/linux/kvm.h | 10 ++++++++++
virt/kvm/arm/arm.c | 24 ++++++++++++++++++++++--
5 files changed, 39 insertions(+), 9 deletions(-)


diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 4df9bb6..fa4cab0 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -751,6 +751,16 @@ struct kvm_ppc_resize_hpt {
#define KVM_S390_SIE_PAGE_OFFSET 1
+ * On arm/arm64, machine type can be used to request the physical
+ * address size for the VM. Bits [7-0] have been reserved for the
+ * PA size shift (i.e, log2(PA_Size)). For backward compatibility,
+ * value 0 implies the default IPA size, which is 40bits.
+ */

This seems like you're allocating quite a lot of bits in a non-extensible
interface to a fairly esoteric parameter. Would it be better to add another
ioctl, or condense the number of sizes you support instead?

As I explained in the other thread, we need the size as soon as the VM
is created. The major challenge is keeping the backward compatibility by
mapping 0 to 40bits. I will give it a thought.