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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, torvalds#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 exhibitis 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 <[email protected]>
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