Re: [kvmtool PATCH 2/2] aarch64: Add support for MTE

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

Hi,

On Mon, Mar 21, 2022 at 03:40:18PM +0000, Vladimir Murzin wrote:
> Hi Alexandru,
> 
> On 3/21/22 3:28 PM, Alexandru Elisei wrote:
> > MTE has been supported in Linux since commit 673638f434ee ("KVM: arm64:
> > Expose KVM_ARM_CAP_MTE"), add support for it in kvmtool.
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> >  arm/aarch32/include/kvm/kvm-arch.h        |  3 +++
> >  arm/aarch64/include/kvm/kvm-arch.h        |  1 +
> >  arm/aarch64/include/kvm/kvm-config-arch.h |  2 ++
> >  arm/aarch64/kvm.c                         | 13 +++++++++++++
> >  arm/include/arm-common/kvm-config-arch.h  |  1 +
> >  arm/kvm.c                                 |  3 +++
> >  6 files changed, 23 insertions(+)
> > 
> > diff --git a/arm/aarch32/include/kvm/kvm-arch.h b/arm/aarch32/include/kvm/kvm-arch.h
> > index bee2fc255a82..5616b27e257e 100644
> > --- a/arm/aarch32/include/kvm/kvm-arch.h
> > +++ b/arm/aarch32/include/kvm/kvm-arch.h
> > @@ -5,6 +5,9 @@
> >  
> >  #define kvm__arch_get_kern_offset(...)	0x8000
> >  
> > +struct kvm;
> > +static inline void kvm__arch_enable_mte(struct kvm *kvm) {}
> > +
> >  #define ARM_MAX_MEMORY(...)	ARM_LOMAP_MAX_MEMORY
> >  
> >  #define MAX_PAGE_SIZE	SZ_4K
> > diff --git a/arm/aarch64/include/kvm/kvm-arch.h b/arm/aarch64/include/kvm/kvm-arch.h
> > index 5e5ee41211ed..9124f6919d0f 100644
> > --- a/arm/aarch64/include/kvm/kvm-arch.h
> > +++ b/arm/aarch64/include/kvm/kvm-arch.h
> > @@ -6,6 +6,7 @@
> >  struct kvm;
> >  unsigned long long kvm__arch_get_kern_offset(struct kvm *kvm, int fd);
> >  int kvm__arch_get_ipa_limit(struct kvm *kvm);
> > +void kvm__arch_enable_mte(struct kvm *kvm);
> >  
> >  #define ARM_MAX_MEMORY(kvm)	({					\
> >  	u64 max_ram;							\
> > diff --git a/arm/aarch64/include/kvm/kvm-config-arch.h b/arm/aarch64/include/kvm/kvm-config-arch.h
> > index 04be43dfa9b2..11250365d8d5 100644
> > --- a/arm/aarch64/include/kvm/kvm-config-arch.h
> > +++ b/arm/aarch64/include/kvm/kvm-config-arch.h
> > @@ -6,6 +6,8 @@
> >  			"Run AArch32 guest"),				\
> >  	OPT_BOOLEAN('\0', "pmu", &(cfg)->has_pmuv3,			\
> >  			"Create PMUv3 device"),				\
> > +	OPT_BOOLEAN('\0', "mte", &(cfg)->has_mte,			\
> > +			"Enable memory tagging extension"),		\
> >  	OPT_U64('\0', "kaslr-seed", &(cfg)->kaslr_seed,			\
> >  			"Specify random seed for Kernel Address Space "	\
> >  			"Layout Randomization (KASLR)"),
> > diff --git a/arm/aarch64/kvm.c b/arm/aarch64/kvm.c
> > index 56a0aedc263d..46548f8ee96e 100644
> > --- a/arm/aarch64/kvm.c
> > +++ b/arm/aarch64/kvm.c
> > @@ -81,3 +81,16 @@ int kvm__get_vm_type(struct kvm *kvm)
> >  
> >  	return KVM_VM_TYPE_ARM_IPA_SIZE(ipa_bits);
> >  }
> > +
> > +void kvm__arch_enable_mte(struct kvm *kvm)
> > +{
> > +	struct kvm_enable_cap cap = {
> > +		.cap = KVM_CAP_ARM_MTE,
> > +	};
> > +
> > +	if (!kvm__supports_extension(kvm, KVM_CAP_ARM_MTE))
> > +		die("MTE capability is not supported");
> > +
> > +	if (ioctl(kvm->vm_fd, KVM_ENABLE_CAP, &cap))
> > +		die_perror("KVM_ENABLE_CAP(KVM_CAP_ARM_MTE)");
> > +}
> > diff --git a/arm/include/arm-common/kvm-config-arch.h b/arm/include/arm-common/kvm-config-arch.h
> > index 5734c46ab9e6..16e8d500a71b 100644
> > --- a/arm/include/arm-common/kvm-config-arch.h
> > +++ b/arm/include/arm-common/kvm-config-arch.h
> > @@ -9,6 +9,7 @@ struct kvm_config_arch {
> >  	bool		virtio_trans_pci;
> >  	bool		aarch32_guest;
> >  	bool		has_pmuv3;
> > +	bool		has_mte;
> >  	u64		kaslr_seed;
> >  	enum irqchip_type irqchip;
> >  	u64		fw_addr;
> > diff --git a/arm/kvm.c b/arm/kvm.c
> > index 80d233f13d0b..f2db93953778 100644
> > --- a/arm/kvm.c
> > +++ b/arm/kvm.c
> > @@ -86,6 +86,9 @@ void kvm__arch_init(struct kvm *kvm, const char *hugetlbfs_path, u64 ram_size)
> >  	/* Create the virtual GIC. */
> >  	if (gic__create(kvm, kvm->cfg.arch.irqchip))
> >  		die("Failed to create virtual GIC");
> > +
> > +	if (kvm->cfg.arch.has_mte)
> > +		kvm__arch_enable_mte(kvm);
> >  }
> 
> Can we enable it unconditionally if KVM_CAP_ARM_MTE is supported like we do for
> PAC and SVE?

I thought about that, the reason I chose to enable it based a kvmtool
command line option, instead of always being enabled if available, is
because of the overhead of sanitising the MTE tags on each stage 2 data
abort. Steven, am I overreacting and that overhead is negligible?

Also, as far as I know, PAC and SVE incur basically no overhead in KVM
until the guest starts to use those features.

Do you have a specific reason for wanting MTE to always be enabled if
available? I'm happy to be convinced to make MTE enabled by default, I
don't have preference either way.

Thanks,
Alex

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