linux-kernel - Re: [RFC PATCH v1] KVM: selftests: Compile s390 tests with -march=z10

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Date:   Fri, 27 Jan 2023 19:12:55 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Nina Schoetterl-Glausch <nsg@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1] KVM: selftests: Compile s390 tests with -march=z10

On 27/01/2023 18.45, Nina Schoetterl-Glausch wrote:
> The guest used in s390 kvm selftests is not be set up to handle all
> instructions the compiler might emit, i.e. vector instructions, leading
> to crashes.
> Limit what the compiler emits to the oldest machine model currently
> supported by Linux.
> 
> Signed-off-by: Nina Schoetterl-Glausch <nsg@...ux.ibm.com>
> ---
> 
> Should we also set -mtune?

I don't think it's really necessary

> Since it are vector instructions that caused the problem here, there
> are some alternatives:
>   * use -mno-vx
>   * set the required guest control bit to enable vector instructions on
>     models supporting them
> 
> -march=z10 might prevent similar issues with other instructions, but I
> don't know if there actually exist other relevant instructions, so it
> could be needlessly restricting.

FWIW, the vector instructions have been introduced with the z13 ... so 
limiting to the zEC12 could be enough.

> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 1750f91dd936..df0989949eb5 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -200,6 +200,9 @@ CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
>   	-I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
>   	-I$(<D) -Iinclude/$(ARCH_DIR) -I ../rseq -I.. $(EXTRA_CFLAGS) \
>   	$(KHDR_INCLUDES)
> +ifeq ($(ARCH),s390)
> +	CFLAGS += -march=z10
> +endif

Starting with z10 sounds sane to me, we still can adjust later if necessary.

Reviewed-by: Thomas Huth <thuth@...hat.com>