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

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Message-ID: <0746a825-ca04-3746-a6ff-5289903a2843@linux.ibm.com>
Date:   Mon, 30 Jan 2023 10:25:32 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Nina Schoetterl-Glausch <nsg@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Huth <thuth@...hat.com>
Subject: Re: [RFC PATCH v1] KVM: selftests: Compile s390 tests with -march=z10

On 1/27/23 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>
> ---

Technically it's a fix but as the tests have been running fine for years 
I'm inclined to put it into a full release rather than a fix rc.

Thanks for taking care of the fix:
Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>

> Should we also set -mtune?
> 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.
> 
> 
>   tools/testing/selftests/kvm/Makefile | 3 +++
>   1 file changed, 3 insertions(+)
> 
> 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
>   
>   no-pie-option := $(call try-run, echo 'int main(void) { return 0; }' | \
>           $(CC) -Werror $(CFLAGS) -no-pie -x c - -o "$$TMP", -no-pie)