linux-kernel - [PATCH] KVM: selftests: x86: Avoid using SSE/AVX instructions

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Message-ID: <20240920154422.2890096-1-vkuznets@redhat.com>
Date: Fri, 20 Sep 2024 17:44:22 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>
Cc: Jan Richter <jarichte@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: selftests: x86: Avoid using SSE/AVX instructions

Some distros switched gcc to '-march=x86-64-v3' by default and while it's
hard to find a CPU which doesn't support it today, many KVM selftests fail
with

  ==== Test Assertion Failure ====
    lib/x86_64/processor.c:570: Unhandled exception in guest
    pid=72747 tid=72747 errno=4 - Interrupted system call
    Unhandled exception '0x6' at guest RIP '0x4104f7'

The failure is easy to reproduce elsewhere with

   $ make clean && CFLAGS='-march=x86-64-v3' make -j && ./x86_64/kvm_pv_test

The root cause of the problem seems to be that with '-march=x86-64-v3' GCC
uses AVX* instructions (VMOVQ in the example above) and without prior
XSETBV() in the guest this results in #UD. It is certainly possible to add
it there, e.g. the following saves the day as well:

diff --git a/tools/testing/selftests/kvm/x86_64/kvm_pv_test.c b/tools/testing/selftests/kvm/x86_64/kvm_pv_test.c
index 78878b3a2725..704668adb3bd 100644
--- a/tools/testing/selftests/kvm/x86_64/kvm_pv_test.c
+++ b/tools/testing/selftests/kvm/x86_64/kvm_pv_test.c
@@ -82,8 +82,17 @@ static void test_hcall(struct hcall_data *hc)

 static void guest_main(void)
 {
+	uint64_t cr4, xcr0;
        int i;

+	cr4 = get_cr4();
+	cr4 |= X86_CR4_OSXSAVE;
+	set_cr4(cr4);
+
+	xcr0 = xgetbv(0);
+	xcr0 |= XFEATURE_MASK_SSE | XFEATURE_MASK_YMM;
+	xsetbv(0x0, xcr0);
+
        for (i = 0; i < ARRAY_SIZE(msrs_to_test); i++) {
                test_msr(&msrs_to_test[i]);
        }

but this needs to be made conditional depending on the compilation target
and added to all selftests. Slap a band-aid on the problem by forcing
'-march=x86-64-v2' in Makefile.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 tools/testing/selftests/kvm/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 48d32c5aa3eb..3f1b24ed7245 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -238,6 +238,7 @@ CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
 	-fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
 	-I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
 	-I$(<D) -Iinclude/$(ARCH_DIR) -I ../rseq -I.. $(EXTRA_CFLAGS) \
+	-march=x86-64-v2 \
 	$(KHDR_INCLUDES)
 ifeq ($(ARCH),s390)
 	CFLAGS += -march=z10
-- 
2.46.0