linux-kernel - [PATCH v2 03/13] KVM: selftests: Fudge around an apparent gcc bug in arm64's PMU test

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Message-ID: <20240911204158.2034295-4-seanjc@google.com>
Date: Wed, 11 Sep 2024 13:41:48 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Anup Patel <anup@...infault.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, James Houghton <jthoughton@...gle.com>
Subject: [PATCH v2 03/13] KVM: selftests: Fudge around an apparent gcc bug in
 arm64's PMU test

Use u64_replace_bits() instead of u64p_replace_bits() to set PMCR.N in
arm64's vPMU counter access test to fudge around what appears to be a gcc
bug.  With the recent change to have vcpu_get_reg() return a value in lieu
of an out-param, some versions of gcc completely ignore the operation
performed by set_pmcr_n(), i.e. ignore the output param.

The issue is most easily observed by making set_pmcr_n() noinline and
wrapping the call with printf(), e.g. sans comments, for this code:

  printf("orig = %lx, next = %lx, want = %lu\n", pmcr_orig, pmcr, pmcr_n);
  set_pmcr_n(&pmcr, pmcr_n);
  printf("orig = %lx, next = %lx, want = %lu\n", pmcr_orig, pmcr, pmcr_n);

gcc-13 generates:

 0000000000401c90 <set_pmcr_n>:
  401c90:       f9400002        ldr     x2, [x0]
  401c94:       b3751022        bfi     x2, x1, #11, #5
  401c98:       f9000002        str     x2, [x0]
  401c9c:       d65f03c0        ret

 0000000000402660 <test_create_vpmu_vm_with_pmcr_n>:
  402724:       aa1403e3        mov     x3, x20
  402728:       aa1503e2        mov     x2, x21
  40272c:       aa1603e0        mov     x0, x22
  402730:       aa1503e1        mov     x1, x21
  402734:       940060ff        bl      41ab30 <_IO_printf>
  402738:       aa1403e1        mov     x1, x20
  40273c:       910183e0        add     x0, sp, #0x60
  402740:       97fffd54        bl      401c90 <set_pmcr_n>
  402744:       aa1403e3        mov     x3, x20
  402748:       aa1503e2        mov     x2, x21
  40274c:       aa1503e1        mov     x1, x21
  402750:       aa1603e0        mov     x0, x22
  402754:       940060f7        bl      41ab30 <_IO_printf>

with the value stored in [sp + 0x60] ignored by both printf() above and
in the test proper, resulting in a false failure due to vcpu_set_reg()
simply storing the original value, not the intended value.

  $ ./vpmu_counter_access
  Random seed: 0x6b8b4567
  orig = 3040, next = 3040, want = 0
  orig = 3040, next = 3040, want = 0
  ==== Test Assertion Failure ====
    aarch64/vpmu_counter_access.c:505: pmcr_n == get_pmcr_n(pmcr)
    pid=71578 tid=71578 errno=9 - Bad file descriptor
       1	0x400673: run_access_test at vpmu_counter_access.c:522
       2	 (inlined by) main at vpmu_counter_access.c:643
       3	0x4132d7: __libc_start_call_main at libc-start.o:0
       4	0x413653: __libc_start_main at ??:0
       5	0x40106f: _start at ??:0
    Failed to update PMCR.N to 0 (received: 6)

Somewhat bizarrely, gcc-11 also exhibits the same behavior, but only if
set_pmcr_n() is marked noinline, whereas gcc-13 fails even if set_pmcr_n()
is inlined in its sole caller.

All signs point to this being a gcc bug, as clang doesn't exhibit the same
issue, the code generated by u64p_replace_bits() is correct, and the error
is somewhat transient, e.g. varies between gcc versions and depends on
surrounding code.

For now, work around the issue to unblock the vcpu_get_reg() cleanup, and
because arguably using u64_replace_bits() makes the code a wee bit more
intuitive.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c b/tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c
index 30d9c9e7ae35..74da8252b884 100644
--- a/tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c
+++ b/tools/testing/selftests/kvm/aarch64/vpmu_counter_access.c
@@ -45,11 +45,6 @@ static uint64_t get_pmcr_n(uint64_t pmcr)
 	return FIELD_GET(ARMV8_PMU_PMCR_N, pmcr);
 }
 
-static void set_pmcr_n(uint64_t *pmcr, uint64_t pmcr_n)
-{
-	u64p_replace_bits((__u64 *) pmcr, pmcr_n, ARMV8_PMU_PMCR_N);
-}
-
 static uint64_t get_counters_mask(uint64_t n)
 {
 	uint64_t mask = BIT(ARMV8_PMU_CYCLE_IDX);
@@ -484,13 +479,12 @@ static void test_create_vpmu_vm_with_pmcr_n(uint64_t pmcr_n, bool expect_fail)
 	vcpu = vpmu_vm.vcpu;
 
 	pmcr_orig = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_PMCR_EL0));
-	pmcr = pmcr_orig;
 
 	/*
 	 * Setting a larger value of PMCR.N should not modify the field, and
 	 * return a success.
 	 */
-	set_pmcr_n(&pmcr, pmcr_n);
+	pmcr = u64_replace_bits(pmcr_orig, pmcr_n, ARMV8_PMU_PMCR_N);
 	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_PMCR_EL0), pmcr);
 	pmcr = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_PMCR_EL0));
 
-- 
2.46.0.598.g6f2099f65c-goog