Re: [PULL 04/29] arm/kvm: add support for MTE

[Cornelia Huck <cohuck@redhat.com>]

On Fri, May 19 2023, Peter Maydell <peter.maydell@linaro.org> wrote:

> On Fri, 19 May 2023 at 14:31, Peter Maydell <peter.maydell@linaro.org> wrote:
>>
>> On Fri, 19 May 2023 at 13:55, Alex Bennée <alex.bennee@linaro.org> wrote:
>> >
>> >
>> > Peter Maydell <peter.maydell@linaro.org> writes:
>> >
>> > > From: Cornelia Huck <cohuck@redhat.com>
>> > >
>> > > 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.
>> > >
>> > > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>> > > Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>> > > Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>> > > Message-id: 20230428095533.21747-2-cohuck@redhat.com
>> > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>> >
>> > I bisected to this commit which causes a segfault on one of my test
>> > kernels (6.3.2 arm64):
>> >
>> >   ➜  ag MTE .config
>> >   486:CONFIG_ARM64_AS_HAS_MTE=y
>> >   487:CONFIG_ARM64_MTE=y
>> >   2949:CONFIG_WLAN_VENDOR_ADMTEK=y
>> >   3573:# CONFIG_I2C_SIMTEC is not set
>> >   5278:# CONFIG_DRM_PANEL_TPO_TD043MTEA1 is not set
>> >   9749:CONFIG_ARCH_USE_MEMTEST=y
>> >   9750:CONFIG_MEMTEST=y

Sigh, this patch seems to be cursed :( Apologies for the fallout.

(I'm wondering what makes this pop up, maybe the CONFIG_MEMTEST?)

>>
>> Try this entirely untested patch?
>>
>> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
>> index f6a88e52ac2..f350661a928 100644
>> --- a/target/arm/cpu.c
>> +++ b/target/arm/cpu.c
>> @@ -1593,6 +1593,15 @@ static void arm_cpu_realizefn(DeviceState *dev,
>> Error **errp)
>>          }
>>      }
>>
>> +    /*
>> +     * For TCG, we can only present MTE to the guest if the board gave us
>> +     * tag RAM. Set has_mte appropriately so code below doesn't need to
>> +     * care whether we're TCG or KVM when deciding if MTE is present.
>> +     */
>> +    if (tcg_enabled() || qtest_enabled()) {
>> +        cpu->has_mte = cpu->tag_memory != NULL;
>> +    }
>> +
>>      if (!tcg_enabled() && !qtest_enabled()) {
>>          /*
>>           * We assume that no accelerator except TCG (and the "not really an
>
> Hmm, actually I don't think that's the only fix needed. It's OK for
> TCG, but for KVM I can't see anywhere in the code that ever sets
> has_mte to false. We default it to on in the cpu.c code, but
> then the board code only sets it to true if MTE is enabled
> (via kvm_arm_enable_mte()).

Hrm, do we need explicit init of this field? Probably needless to say
that I didn't hit this problem in any of my tests... I suspect that only
specific kernels hit this?