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

[Ganapatrao Kulkarni]

Hi Peter,


[Apologies for the delayed response]

On 16-07-2024 09:15 pm, Peter Maydell wrote:
> On Tue, 9 Jul 2024 at 07:05, 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>
> > ---
>
> In target/arm/cpu.c:arm_cpu_realizefn() there is this code:
>
>      if (cpu_isar_feature(aa64_mte, cpu)) {
>          /*
>           * The architectural range of GM blocksize is 2-6, however qemu
>           * doesn't support blocksize of 2 (see HELPER(ldgm)).
>           */
>          if (tcg_enabled()) {
>              assert(cpu->gm_blocksize >= 3 && cpu->gm_blocksize <= 6);
>          }
>
> #ifndef CONFIG_USER_ONLY
>          /*
>           * If we do not have tag-memory provided by the machine,
>           * reduce MTE support to instructions enabled at EL0.
>           * This matches Cortex-A710 BROADCASTMTE input being LOW.
>           */
>          if (cpu->tag_memory == NULL) {
>              cpu->isar.id_aa64pfr1 =
>                  FIELD_DP64(cpu->isar.id_aa64pfr1, ID_AA64PFR1, MTE, 1);
>          }
> #endif
>      }
>
> With this patch, for KVM we will end up going through the
> "squash ID_AA64PFR1_EL1.MTE to 1" codepath, because KVM doesn't
> set cpu->tag_memory and this is still using that as its check.

I looked at this function and it seems we are not entering this function 
for KVM boot. I do see -DCONFIG_USER_ONLY added to make files.

Also Linux kernel wont detect/enable MTE until unless the 
ID_AA64PFR1_EL1.MTE value is 2(b0010) and above.

> More generally, how does the enabling of the MTE KVM cap
> interact with the ID_AA64PFR1_EL1 value that we read from
> the host in kvm_arm_get_host_cpu_features() ? We care that we
> have the right ID register values because we use ID field
> checks to determine whether the vcpu has a feature or not,
> even in the KVM case.
>
> Since Cornelia first wrote the patch this is based on, we've
> landed gdbstub support for MTE (so gdb can find out which
> addresses in the memory map have tags and read and write
> those tags). So I think the KVM MTE support now also needs to
> handle that. (See aarch64_cpu_register_gdb_commands() in
> target/arm/gdbstub64.c.)

Ok sure, I will go through this file to add/update MTE part


Thanks,
Ganapat