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Date: Wed, 4 Jul 2018 23:03:10 +0100 From: Suzuki K Poulose <suzuki.poulose@....com> To: Will Deacon <will.deacon@....com> Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu, james.morse@....com, marc.zyngier@....com, cdall@...nel.org, eric.auger@...hat.com, julien.grall@....com, catalin.marinas@....com, punit.agrawal@....com, qemu-devel@...gnu.org, Peter Maydel <peter.maydell@...aro.org>, Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com> Subject: Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM 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@....com> >> Cc: Christoffer Dall <cdall@...nel.org> >> Cc: Peter Maydel <peter.maydell@...aro.org> >> Cc: Paolo Bonzini <pbonzini@...hat.com> >> Cc: Radim Krčmář <rkrcmar@...hat.com> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com> >> --- >> 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. >> + */ >> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK 0xff >> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT(x) \ >> + ((x) & KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK) > > 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. Suzuki
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