Re: [PATCH V2] arm/kvm: add support for MTE

[Cornelia Huck]

On Thu, Sep 12 2024, Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> 
wrote:

> Extend the 'mte' property for the virt machine to cover KVM as
> well. For KVM, we don't allocate tag memory, but instead enable
> the capability.
>
> If MTE has been enabled, we need to disable migration, as we do not
> yet have a way to migrate the tags as well. Therefore, MTE will stay
> off with KVM unless requested explicitly.
>
> This patch is rework of commit b320e21c48ce64853904bea6631c0158cc2ef227
> which broke TCG since it made the TCG -cpu max
> report the presence of MTE to the guest even if the board hadn't
> enabled MTE by wiring up the tag RAM. This meant that if the guest
> then tried to use MTE QEMU would segfault accessing the
> non-existent tag RAM.
>
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
> ---
> Changes since V1:
>       Added code to enable MTE before reading register
> id_aa64pfr1 (unmasked MTE bits).
>
> This patch is boot tested on ARM64 with KVM and on X86 with TCG for mte=on
> and default case(i.e, no mte).
>
>  hw/arm/virt.c        | 72 ++++++++++++++++++++++++++------------------
>  target/arm/cpu.c     |  7 +++--
>  target/arm/cpu.h     |  2 ++
>  target/arm/kvm.c     | 59 ++++++++++++++++++++++++++++++++++++
>  target/arm/kvm_arm.h | 19 ++++++++++++
>  5 files changed, 126 insertions(+), 33 deletions(-)
>

(...)

> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> index 849e2e21b3..29865609fb 100644
> --- a/target/arm/kvm.c
> +++ b/target/arm/kvm.c
> @@ -39,6 +39,7 @@
>  #include "hw/acpi/acpi.h"
>  #include "hw/acpi/ghes.h"
>  #include "target/arm/gtimer.h"
> +#include "migration/blocker.h"
>  
>  const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
>      KVM_CAP_LAST_INFO
> @@ -119,6 +120,21 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t 
> *cpus_to_try,
>      if (vmfd < 0) {
>          goto err;
>      }
> +
> +    /*
> +     * MTE bits of register id_aa64pfr1 are masked if MTE is
> +     * not enabled and required to enable before VCPU
> +     * is created. Hence enable MTE(if supported) before VCPU
> +     * is created to read unmasked MTE bits.
> +     */

Maybe

"KVM will mask the MTE bits in id_aa64pfr1 unless the VMM has enabled
the MTE KVM capability, so do it here for probing."

?

> +    if (kvm_arm_mte_supported()) {
> +        KVMState kvm_state;
> +
> +        kvm_state.fd = kvmfd;
> +        kvm_state.vmfd = vmfd;
> +        kvm_vm_enable_cap(&kvm_state, KVM_CAP_ARM_MTE, 0);
> +    }
> +
>      cpufd = ioctl(vmfd, KVM_CREATE_VCPU, 0);
>      if (cpufd < 0) {
>          goto err;